Representation of the complex relationship between HLA susceptibility and psoriatic arthritis. The areas on the diagram are not exactly drawn to scale. The right side of the diagram depicts the presence of the Cw*0602 allele in healthy people, its strong association with cutaneous psoriasis susceptibility, and the fact that approximately 40% of those with psoriasis lack Cw*0602. The left side depicts the almost complete association of B*27 with ankylosing spondylitis. Psoriatic arthritis, which includes psoriasis plus the musculoskeletal phenotype, is shown as the thick rimmed circle. Both Cw*0602 and B*27 alleles contribute independently to psoriatic arthritis susceptibility. HLA, human leukocyte antigen.