Although the exact cause of AS is unknown, we do know that genetics play a key role in AS. Most individuals who have AS also have a gene that produces a "genetic marker" - in this case, a protein - called HLA-B27. This marker is found in over 95% of people in the Caucasian population with AS (the association between ankylosing spondylitis and HLA-B27 varies greatly between ethnic and racial groups, see our AS Diagnosis section for more information). It is important to note, however, that you do not have to be HLA-B27 positive to have AS. Also, a majority of the people with this marker never contract ankylosing spondylitis.

Scientists suspect that other genes, along with a triggering environmental factor, such as a bacterial infection, are needed to trigger AS in susceptible people. HLA-B27 probably accounts for about 40% of the overall risk, but then there are other genes working in concert with B27. There are probably five or six genes involved in susceptibility toward AS. It is thought that perhaps AS starts when the defenses of the intestines start breaking down and bacteria from the intestines pass into the bloodstream directly into the region where the sacroiliac joints are located.