Back Pain : What causes ankylosing spondylitis?

What causes ankylosing spondylitis?
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The cause is not known. However, there is a strong genetic (hereditary) tendency. For example, there is a strong association with a gene called HLA-B27. About 9 in 10 people with AS have this gene compared with less than 1 in 10 in the general population. However, having this gene does not mean that you will automatically get AS. Many people with this gene do not develop AS. Also, some people develop AS who do not have this gene. It is thought that something may trigger AS to develop in people who have an inherited tendency to develop AS, and the gene HLA-B27 provides a strong tendency. However, the trigger is not known and other genes and factors may also be involved in causing AS.