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Doctors' Answers to "Frequently Asked Questions" - Ankylosing Spondylitis
Answer: The tendency to acquire ankylosing spondylitis is inherited, but for some reason not all
in a family will have this problem. It is not clear whether this is a type of autoimmune
connective tissue disease or related to other problems with the inherited genes. AS is
commonly seen with the histocompatibility antigen HLA-B27. Over 90% of patients will have
this antigen (inherited). However, if you inherit the antigen only about 2% of these
individuals will develop the disease. Twins run about 50-65%, so there is something besides
inheritance which causes the disease.