Psoriatic arthritis (PsoA) is a common comorbidity of psoriasis. Despite the high prevalence, arthritic symptoms often remain undiagnosed. In addition to the CASPAR classification (“Classification Criteria for Psoriatic Arthritis”), there are certain genetic factors which are associated with joint involvement. The human leukocyte antigen (HLA) system plays an important role, with HLA-B-27, B-39, and B-07 in particular associated with PsoA.