The HLAs are characterized by an extensive degree of allelic polymorphism; strong associations between hypersensitivity or autoimmune disease and certain HLA alleles have been reported, for instance, HLA-B*27 and ankylosing spondylitis (Brewerton et al. 1973), and HLA-B*1502 and carbamazepine-induced Stevens-Johnson syndrome (Chung et al. 2004). The gene discussed is HLA-B; the disease is ankylosing spondylitis.