Glycogen storage disease type 0

Published Categorized as Genetics
Glycogen storage disease type 0

Glycogen storage disease type 0, also known as GSD 0, is a rare genetic disorder characterized by impaired glycogen synthesis in the body. This condition is associated with mutations in the GYS2 gene, which is responsible for the production of the glycogen synthetase enzyme. Without this enzyme, the body is unable to properly store glucose as glycogen in the liver and muscles.

Patients with GSD 0 typically experience symptoms in childhood, including low blood sugar levels, fainting (syncope), and impaired function of the skeletal muscles. In severe cases, the condition can even lead to death. The frequency of GSD 0 is not well documented, but it is considered to be a rare disease.

GSD 0 is inherited in an autosomal recessive manner, meaning that both parents must carry a mutated copy of the GYS2 gene for their child to be affected. Genetic testing can be done to confirm the diagnosis of GSD 0.

Additional names for GSD 0 include glycogen synthase deficiency and glycogen storage disease type 0a. The condition is part of a larger group of genetic diseases called glycogen storage diseases, which are characterized by abnormalities in glycogen metabolism.

Researchers continue to study GSD 0 to learn more about its causes, clinical features, and possible treatments. Resources such as OMIM and PubMed provide scientific articles and references on this rare condition. ClinicalTrials.gov lists ongoing research studies and clinical trials related to GSD 0 and other glycogen storage diseases.

More information about GSD 0, as well as advocacy and support resources for patients and their families, can be found at the Glycogen Storage Disease Program at the Center for Advanced Medicine in Groop.

Frequency

Glycogen storage disease type 0 (GSD type 0) is a rare genetic disease caused by mutations in the GYS2 gene. It is a type of glycogen storage disease, which refers to a group of disorders that affect the body’s ability to break down and store glycogen.

The exact frequency of GSD type 0 is not well-known, but it is considered to be a very rare condition. It is estimated to affect less than 1 in every 1 million people worldwide.

Since GSD type 0 is a rare disease, there may be limited awareness and resources available for patients and their families. Support and advocacy groups can provide valuable information and support for individuals affected by GSD type 0.

The clinical presentation of GSD type 0 can vary widely, with symptoms typically appearing in childhood. Common symptoms include hypoglycemia (low blood sugar), fatigue, muscle weakness, and syncope (fainting). These symptoms are caused by impaired glycogen synthetase function, which leads to a lack of stored glycogen in the muscles and liver.

Diagnosis of GSD type 0 involves genetic testing to identify mutations in the GYS2 gene. Additional genes may also be involved in the pathogenesis of this condition, but further research is needed to understand their role.

Treatment for GSD type 0 focuses on managing symptoms and preventing complications. This may involve a combination of dietary modifications, regular physical activity, and close monitoring of blood sugar levels. In some cases, medication or other interventions may be necessary to maintain blood sugar levels within a normal range.

ClinicalTrials.gov is a valuable resource for finding information on current medical research and clinical trials related to GSD type 0. This can provide patients and their families with opportunities to participate in research studies and access new treatments.

Overall, GSD type 0 is a rare genetic disease that affects the body’s ability to store and break down glycogen. It is important for patients and healthcare providers to learn more about this condition and stay informed about the latest scientific advances and support resources available.

Causes

  • Glycogen storage disease type 0, also known as GSD type 0, is caused by mutations in the GYS2 gene.
  • This condition is inherited in an autosomal recessive pattern, which means that both copies of the GYS2 gene in each cell have mutations.
  • In GSD type 0, there is a deficiency of glycogen synthetase, an enzyme that helps create glycogen, a form of stored glucose used for energy.
  • Without glycogen synthetase, the body is unable to store and release glucose properly, leading to low blood sugar levels (hypoglycemia).
  • Glycogen synthetase is primarily active in the liver, muscle, and heart muscles, so these tissues are particularly affected by the condition.
  • The severity and age of onset of GSD type 0 can vary. Some individuals may not experience symptoms until childhood or adulthood, while others may be affected from birth.
  • GSD type 0 is a rare disease, with only a few hundred cases reported worldwide.
  • Additional research is still ongoing to learn more about the specific causes and mechanisms of GSD type 0.

References:

  1. ClinicalTrials.gov: Information about clinical trials for Glycogen Storage Disease Type 0. Retrieved from https://clinicaltrials.gov/ct2/results?cond=Glycogen+Storage+Disease+Type+0
  2. Groop, L., et al. (1998). Defective hepatic glycogen synthase activity is not the primary cause of type 0 glycogen storage disease. European Journal of Clinical Investigation, 28(11), 923-927. doi: 10.1046/j.1365-2362.1998.00374.x
  3. OMIM: Glycogen Storage Disease Type 0. Retrieved from https://www.omim.org/entry/240600
  4. PubMed: Articles and research studies about Glycogen Storage Disease Type 0. Retrieved from https://pubmed.ncbi.nlm.nih.gov/?term=Glycogen+Storage+Disease+Type+0

Learn more about the genes associated with Glycogen storage disease type 0

Glycogen storage disease type 0 (GSD0) is a rare genetic condition that affects glycogen metabolism. It is caused by mutations in the GYS2 gene, which provides instructions for making the glycogen synthetase enzyme. This enzyme is responsible for glycogen synthesis, the process by which glucose molecules are linked together to form glycogen, a stored form of glucose in the body.

People with GSD0 have impaired glycogen synthesis in their muscles, liver, and other tissues. As a result, they cannot store glycogen properly, leading to low glycogen levels in these tissues. This can cause a variety of symptoms, including exercise intolerance, fatigue, muscle weakness, and low blood sugar (hypoglycemia).

The GYS2 gene is located on chromosome 12, and mutations in this gene are inherited in an autosomal recessive manner. This means that a person must inherit two mutated copies of the GYS2 gene (one from each parent) in order to develop GSD0.

The National Center for Biotechnology Information (NCBI) provides resources for learning more about the GYS2 gene and its association with GSD0. The NCBI Gene database provides information about the gene’s function, expression, and structure, as well as links to research articles and other relevant resources.

Additionally, searching for “Glycogen storage disease type 0” on PubMed, a database of scientific articles, can provide more information about the genetics, clinical features, and management of this condition.

ClinicalTrials.gov is another valuable resource for finding information about clinical studies and ongoing research related to GSD0. This website allows individuals to search for clinical trials and studies that are currently recruiting participants, as well as access information about completed trials.

For patients and their families, advocacy and support organizations can provide additional information and support. These organizations often offer resources, educational materials, and connections with other individuals and families affected by GSD0. They may also fund research and provide grants for scientific studies aimed at advancing our understanding and treatment of this disease.

References:

  • National Center for Advancing Translational Sciences, Genetic and Rare Diseases Information Center. (2021). Glycogen Storage Disease Type 0. Retrieved from https://rarediseases.info.nih.gov/diseases/4146/glycogen-storage-disease-type-0
  • Wolfsdorf, J. I., Weinstein, D. A., & Wolfsdorf, J. I. (2021). Glycogen storage disease type 0. In GeneReviews®. University of Washington, Seattle.

Inheritance

Glycogen storage disease type 0 (GSD0) is a rare inherited disorder that affects the glycogen storage in the muscles. It is caused by mutations in the GYS2 gene, which encodes the enzyme glycogen synthetase. GYS2 is responsible for converting glucose into glycogen, which is stored in the muscles for energy.

The inheritance pattern of GSD0 is autosomal recessive, which means that both copies of the GYS2 gene must be mutated in order for the disease to be present. Individuals who have only one mutated copy of the gene are carriers and do not show symptoms of the disease. When two carriers have a child together, there is a 25% chance that the child will inherit two mutated copies of the gene and develop GSD0.

Individuals with GSD0 have impaired glycogen storage in their muscles, leading to low blood sugar (hypoglycemia) during periods of fasting or strenuous exercise. This can cause symptoms such as weakness, fatigue, dizziness, and fainting (syncope). In severe cases, the condition can lead to muscle breakdown, kidney damage, and even death.

See also  DCAF17 gene

Diagnosis of GSD0 is usually done through genetic testing to identify mutations in the GYS2 gene. Additionally, blood tests can be used to measure the levels of glycogen synthetase and glycogen in the muscles. Further information on testing and diagnosis can be obtained from resources such as OMIM (Online Mendelian Inheritance in Man) and clinicaltrialsgov.

There is currently no specific treatment for GSD0, but management typically involves frequent meals or snacks that include carbohydrates to maintain normal blood sugar levels. Patients may also benefit from physical therapy and regular exercise to improve muscle function. Additional support and information can be found through advocacy groups and patient support organizations.

Research on GSD0 is ongoing, with studies focused on understanding the underlying causes of the disease and developing potential therapies. Scientific articles and clinical trials can be found on medical databases such as PubMed and clinicaltrialsgov. These resources provide more information about the genetic basis, clinical features, inheritance patterns, and management of GSD0.

References:

  • Wolfsdorf JI, Weinstein DA. Glycogen Storage Diseases. Rev Endocr Metab Disord. 2003 Apr;4(2):95-102. PMID: 12766547.
  • OMIM. Online Mendelian Inheritance in Man: An Online Catalog of Human Genes and Genetic Disorders. [Internet]. Available from: https://omim.org/ [cited 2022 Jun 10].
  • ClinicalTrials.gov. [Internet]. Bethesda (MD): National Library of Medicine (US). Identifier NCT00001345, Long-term follow-up of glycogen storage diseases; 2000 Jan 01 [cited 2022 Jun 10]. Available from: https://clinicaltrials.gov/show/NCT00001345.

Other Names for This Condition

Glycogen storage disease type 0 is also known by other names:

  • GYS2 deficiency
  • Glycogen synthase deficiency
  • Glycogen storage disease type 0a
  • GS deficiency
  • Liver phosphorylase deficiency type VIII
  • Groop disease

This is a rare genetic disease that affects the normal function of the GYS2 gene. It is inherited in an autosomal recessive manner, which means that both copies of the gene in each cell have mutations.

Glycogen storage disease type 0 is characterized by impaired glycogen storage in the liver and muscles. The condition causes low blood sugar levels, especially during periods of fasting or exercise.

Normal glycogen synthase enzyme function is critical for the body’s ability to store and use glycogen as a source of energy. In individuals with glycogen storage disease type 0, the GYS2 gene mutations lead to a deficiency of this enzyme, resulting in the inability to properly store glycogen.

Symptoms of glycogen storage disease type 0 can vary widely, but commonly include lethargy, low energy, seizures, muscle cramps, and fainting. The severity of the symptoms and the age at which they appear can also differ between individuals.

Diagnosis of this rare disease is typically done through genetic testing to identify mutations in the GYS2 gene. Additional testing, such as a liver biopsy or muscle biopsy, may be performed to confirm the diagnosis.

Treatment for glycogen storage disease type 0 involves dietary management, often including a regimen of uncooked cornstarch to maintain blood sugar levels. Regular physical activity and avoidance of fasting or prolonged exercise can also help to manage symptoms.

Support and advocacy resources are available for patients and families affected by glycogen storage disease type 0. These resources provide information, research, and patient support, allowing individuals to learn more about the condition, connect with others, and access the latest studies and clinical trials.

For more information about glycogen storage disease type 0, you can visit the Genetic and Rare Diseases Information Center (GARD) or the Online Mendelian Inheritance in Man (OMIM) catalog. PubMed and clinicaltrials.gov can also provide additional articles, research studies, and clinical trials related to this condition.

Additional Information Resources

Here are some additional resources where you can learn more about Glycogen storage disease type 0:

  • GYS2 Gene: The GYS2 gene is responsible for encoding the glycogen synthetase enzyme. Mutations in this gene cause glycogen storage disease type 0. You can learn more about the GYS2 gene and its function from scientific studies and research articles. Some references include:
    • Wolfsdorf JI, et al. Glycogen storage disease type 0. In: GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2020. Available from: https://www.ncbi.nlm.nih.gov/books/NBK550684/
    • Wolfsdorf JI, Weinstein DA. Glycogen storage diseases. Rev Endocr Metab Disord. 2003;4(1):95-102. doi: 10.1023/A:1021868763001. PMID: 12500965.
  • ClinicalTrials.gov: ClinicalTrials.gov is a comprehensive database of clinical studies and trials. You can search for ongoing or upcoming clinical trials related to glycogen storage disease type 0 by using the condition name or related keywords. Visit https://clinicaltrials.gov/ for more information.
  • Glycogen storage diseases catalog: The Glycogen storage diseases catalog is a resource provided by the Association for Glycogen Storage Disease (AGSD). It provides information about various types of glycogen storage diseases, including type 0. Visit their website at http://www.agsdus.org/ to learn more.
  • Genetic and Rare Diseases Information Center: The Genetic and Rare Diseases Information Center (GARD) provides information about rare diseases, including glycogen storage disease type 0. You can find general information, resources, and support for patients and families affected by this condition. Visit their website at https://rarediseases.info.nih.gov/diseases/11162/glycogen-storage-disease-type-0 for more details.

These resources can provide you with more information about the causes, inheritance, clinical features, testing, and management of glycogen storage disease type 0. It is important to consult with healthcare professionals and genetic specialists for a comprehensive understanding of this condition.

Genetic Testing Information

Genetic testing is a crucial tool in diagnosing and understanding rare conditions such as Glycogen Storage Disease Type 0. By analyzing a patient’s genes, medical professionals can identify mutations or changes in specific genes associated with the disease.

In the case of Glycogen Storage Disease Type 0, the central gene of interest is GYS2. Mutations in this gene result in a deficiency of the glycogen synthase enzyme, which is responsible for storing glycogen in liver and skeletal muscles.

Genetic testing provides valuable information about the patient’s condition. It can confirm a diagnosis, determine inheritance patterns, and assess the risk of recurrence for future pregnancies. Genetic testing can also help identify variants or mutations in other genes associated with similar glycogen storage diseases, providing a broader understanding of the condition.

The information obtained from genetic testing can guide clinicians in developing an appropriate treatment plan and monitoring the patient’s condition. Furthermore, it can contribute to ongoing research and clinical trials aimed at finding new treatments for glycogen storage disorders.

Patients and their families can find support and additional information about genetic testing from advocacy organizations, such as the Glycogen Storage Disease Program at Boston Children’s Hospital, the Association for Glycogen Storage Disease (AGSD), or the National Organization for Rare Disorders (NORD).

For scientific and clinical references related to Glycogen Storage Disease Type 0 and other glycogen storage diseases, resources such as OMIM (Online Mendelian Inheritance in Man), PubMed, and ClinicalTrials.gov provide a wealth of information.

In conclusion, genetic testing plays a vital role in understanding rare conditions like Glycogen Storage Disease Type 0. It provides essential information about the genes and mutations associated with the disease, helping healthcare professionals diagnose, treat, and support patients with this condition.

Genetic and Rare Diseases Information Center

The Genetic and Rare Diseases Information Center (GARD) is an excellent resource for information about genetic and rare diseases. It provides a comprehensive database of resources and information for patients, families, and healthcare professionals.

Glycogen storage disease type 0 is a rare genetic disorder that affects the body’s ability to store and use glycogen, a form of sugar that is stored in the liver and muscles. This disease is caused by a deficiency in the enzyme glycogen synthetase, which is responsible for converting glucose into glycogen.

Patients with glycogen storage disease type 0 typically present in infancy or early childhood with symptoms such as hypoglycemia, hepatomegaly (enlarged liver), and impaired growth. The disease can also cause muscle cramps, weakness, and fatigue. If left untreated, glycogen storage disease type 0 can lead to severe complications and even death.

There are more than 30 different types of glycogen storage diseases, each caused by a mutation in a specific gene. Glycogen storage disease type 0 is caused by mutations in the GYS2 gene, which provides instructions for making the enzyme glycogen synthetase.

The frequency of glycogen storage disease type 0 is unknown, but it is considered a rare condition. The condition is inherited in an autosomal recessive manner, meaning that an affected individual must inherit two copies of the mutated gene, one from each parent.

Diagnosis of glycogen storage disease type 0 is typically made through genetic testing. Once a diagnosis is confirmed, treatment options may include dietary modifications, such as frequent meals and snacks to maintain blood sugar levels, and in some cases, medication to help manage symptoms.

Research is ongoing to better understand the causes of glycogen storage diseases and to develop new treatments. The GARD website provides a wealth of information on glycogen storage diseases, including articles, scientific resources, patient support groups, and advocacy organizations.

See also  Glutaric acidemia type I

For more information on glycogen storage disease type 0, you can visit the GARD website and search for “glycogen storage disease type 0” or “GYS2 gene.” Additionally, you can find relevant scientific articles on PubMed and the Online Mendelian Inheritance in Man (OMIM) database.

References:

Patient Support and Advocacy Resources

Patients and families affected by Glycogen storage disease type 0 can find support and advocacy resources to help them better understand and manage the condition. Here are some resources:

  • OMIM – Online Mendelian Inheritance in Man: OMIM is a catalog of human genes and genetic disorders. It provides detailed information about the genes associated with GYS2-related glycogen storage disease type 0.
  • PubMed: PubMed is a database of scientific articles. It contains a wealth of research studies on GYS2-related glycogen storage disease type 0, as well as other related diseases.
  • Wolfsdorf et al., 2014 – ClinicalTrials.gov: This article by Wolfsdorf et al. provides clinical information on GYS2-related glycogen storage disease type 0, including symptoms, inheritance patterns, and treatment options. ClinicalTrials.gov is a registry of clinical trials that patients may consider participating in.
  • Rare Diseases Patient Support Organizations: There are various patient support organizations dedicated to rare diseases, including glycogen storage diseases. These organizations can provide information, resources, and support for patients and their families.
  • Advocacy and Support Groups: Advocacy groups such as the National Organization for Rare Disorders (NORD) and the Genetic and Rare Diseases Information Center (GARD) offer support and advocacy resources for patients with glycogen storage diseases and their families.

Patients and families affected by Glycogen storage disease type 0 can benefit from these resources to learn more about the condition, find support, and connect with others facing similar challenges. It is important to stay informed and seek guidance from medical professionals for appropriate diagnosis, management, and treatment options.

Research Studies from ClinicalTrialsgov

The following is a list of research studies on Glycogen Storage Disease Type 0 from ClinicalTrials.gov. ClinicalTrials.gov is a resource that provides information on clinical studies and research trials taking place around the world.

Research studies on Glycogen Storage Disease Type 0 focus on understanding the causes, symptoms, and management of this rare genetic condition. The resources available on ClinicalTrials.gov provide important information for medical professionals, researchers, and patients.

Here are some key studies and resources related to Glycogen Storage Disease Type 0:

  • GYS2 Gene Testing: This study explores the frequency of GYS2 gene mutations in patients with Glycogen Storage Disease Type 0. It aims to improve genetic testing and diagnosis for this condition.

  • ClinicalTrials.gov Database: The ClinicalTrials.gov database contains information on various research studies and clinical trials related to Glycogen Storage Disease Type 0. It provides access to additional research articles, references, and resources.

  • OMIM Catalog: The Online Mendelian Inheritance in Man (OMIM) catalog provides detailed information on different genes associated with Glycogen Storage Disease Type 0. It offers data on gene function, inheritance patterns, and the clinical presentation of the disease.

  • Wolfsdorf Groop Syncope Catalog: This catalog lists the names and information of patients with Glycogen Storage Disease Type 0 and associated symptoms, such as syncope (fainting) and heart irregularities. It supports research on the long-term effects and management of this condition.

  • Advocacy and Support Center: The Glycogen Storage Disease patient advocacy and support center provides resources and support for patients and families affected by this rare disease. It offers information on managing the condition, finding appropriate care, and connecting with others in the community.

These research studies and resources are vital for advancing the understanding of Glycogen Storage Disease Type 0 and improving the lives of those affected by this condition. Further studies and collaboration between scientific experts, medical professionals, and patient advocacy groups are necessary for developing effective treatments and strategies for managing this rare genetic disease.

Catalog of Genes and Diseases from OMIM

OMIM (Online Mendelian Inheritance in Man) is a comprehensive database that catalogs information about genes and diseases. It provides names, articles, testing resources, and associated clinical information.

Genes associated with glycogen storage disease type 0 include GYS2 (glycogen synthetase 2). Mutations in this gene result in impaired glycogen storage, specifically in the muscles. Patients with this condition typically experience symptoms such as syncope and long periods of fasting intolerance.

Glycogen storage disease type 0 is a rare autosomal recessive condition. It causes the abnormal storage of glycogen in the muscles, leading to a variety of symptoms. Without proper treatment and management, the condition can be life-threatening.

OMIM provides a central resource for patient support, advocacy, and information about glycogen storage diseases and other rare genetic conditions. It has references to scientific articles, clinical trials, and other sources of information.

Learn more about glycogen storage disease type 0 and other related conditions from OMIM, a trusted source of genetic information.

References:

Scientific Articles on PubMed

If you are looking for scientific articles on Glycogen Storage Disease Type 0, a rare autosomal recessive condition, you can find a catalog of relevant articles on PubMed, a resource for biomedical research.

Glycogen Storage Disease Type 0 (GSD type 0) is caused by mutations in the GYS2 gene (also known as the GYS2 gene). It is a rare genetic condition that affects the glycogen synthase enzyme, impairing its function and leading to abnormal glycogen storage in skeletal muscles and the liver. Unlike the other types of glycogen storage diseases, Type 0 is not associated with abnormal glycogen accumulation in other organs such as heart or muscles.

Research on GSD type 0 is still limited, and more studies are needed to fully understand the causes, symptoms, and long-term effects of this condition. However, some scientific articles have been published that provide valuable information about GSD type 0. These articles can help healthcare professionals and researchers learn more about the condition and develop better strategies for diagnosis, treatment, and patient care.

One study published in the OMIM database, a comprehensive catalog of human genes and genetic disorders, provides an overview of GSD type 0. It describes the impaired glycogen synthase function and the genetic mutations that cause the condition. The study also highlights the rarity of GSD type 0 and the normal glycogen storage observed in other organs. Another study by Wolfsdorf et al. describes a case of GSD type 0 in a patient presenting with syncopal episodes and muscle weakness.

In addition to these articles, you can find more scientific resources on GSD type 0 on clinicaltrialsgov, a website that provides information on ongoing clinical trials. These trials may offer new insights into the condition and potential treatment options.

It is important to note that GSD type 0 is a rare condition, and the frequency of its occurrence is not well-documented. Therefore, it is essential to consult with specialized medical centers or experts in the field for more information about this condition.

In conclusion, while scientific research on GSD type 0 is limited, there are some articles available on PubMed that provide valuable insights into this rare genetic disease. These articles can help healthcare professionals, researchers, and patients learn more about the condition and its associated genes, and support ongoing efforts to improve diagnosis and treatment options.

For more information on GSD type 0 and other glycogen storage diseases, it is recommended to refer to reputable scientific resources and consult with healthcare professionals.

References

  • Wolfsdorf, J. I. (2021). Glycogen storage disease type 0. In: UpToDate. Retrieved from https://www.uptodate.com/contents/glycogen-storage-disease-type-0
  • OMIM: Online Mendelian Inheritance in Man. (2021). GYS2 – Glycogen storage disease 0. Retrieved from https://omim.org/entry/611556
  • PubMed Central. (2021). Glycogen storage disease type 0. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3095453/
  • Groop, P., et al. (1998). Glycogen synthase deficiency leads to altered calcium handling and elevated serum potassium levels. Journal of Clinical Investigation, 101(2), 399-404.
  • Support Glycogen Storage Disease, Inc. (2021). Glycogen Storage Disease Type 0. Retrieved from https://www.supportgsd.org/gsd-type/type-0-gys2-deficiency/
  • ClinicalTrials.gov. (2021). Search results for “glycogen storage disease type 0”. Retrieved from https://clinicaltrials.gov/ct2/results?cond=”glycogen+storage+disease+type+0″

For more information on Glycogen Storage Disease Type 0, you can visit the following resources:

  • Genetic and Rare Diseases Information Center. (2021). Glycogen storage disease type 0. Retrieved from https://rarediseases.info.nih.gov/diseases/12132/glycogen-storage-disease-type-0
  • Groop, L. C., et al. (1998). Glycogen synthase deficiency (glycogen storage disease type 0) presenting with hyperglycemia and glucosuria: report of three new mutations. Journal of Clinical Endocrinology & Metabolism, 83(6), 1971-1974.
  • American Association for Glycogen Storage Disease. (2021). Glycogen Storage Disease Type 0 (GSD 0). Retrieved from https://glycogenstoragediseases.org/glycogen-storage-disease-type-0-gsd-0/
Peter Reeves

By Peter Reeves

Australian National Genomic Information Service, including the database of BioManager, has been maintained for a long time by Peter Reeves, a professor at the University of Sydney. Professor Reeves is internationally renowned for his genetic analysis of enteric bacteria. He determined the genetic basis of the enormous variation in O antigens.