Twenty-five years ago, HLA-B27 was recognized as a new HLA specificity. It is present throughout Eurasia but is virtually absent among the genetically unmixed native populations of South America, the Bantus and Sans (Bushmen) of equatorial and southern Africa, and the aborigines of Australia. B27 is a serologically determined specificity that encompasses nine different allotypes (subtypes), B*2701 to B*2709. Ankylosing spondylitis or related spondyloarthropathies thus far have been documented in individuals with any of the following five subtypes: B*2701, B*2702, B*2704, B*2705, or B*2707.