HLA-B27 is a genetic marker. In people with inflammatory arthritis of the spine and joints (not osteoarthritis), a positive HLA-B27 test is associated with the presence of one of a group of diseases called seronegative spondyloarthropathies. This includes diseases such as ankylosing spondylitis (AS), psoriatic arthritis, and Reiter's syndrome (also called reactive arthritis). HLA-B27 is present in about 90% of people with AS, but the gene can also be seen in people with no sign of arthritis or inflammation.