Other than sacroiliitis and enthesitis, the HLA-B27 is helpful in establishing the diagnosis of Ankylosing Spondylitis.
The gene carrier rate is about 7% in the general population. It is estimated that about 7% of people with this gene will develop a disease in the Spondyloarthritis stable in their lifetime. For carriers with a first degree relative with AS, the likelihood of developing disease goes up to 25%.
While this is probably the best known genetic association with human disease, the exact pathogenic mechanism is not entirely worked out.
Given the diverse manifestations of the disease subtypes in the Spondyloarthritis stable, diagnosis does not come easy. It is still a work in progress.
HLA-B27 only accounts for about a quarter of the genetic predisposition to AS. Other genes may play a role. One such genetic association is described in this paper.