Psoriatic arthritis (PsA) is a chronic inflammatory disease that primarily invades the musculoskeletal system and skin. 1 Usually, besides the arthritis damage, including arthritis, enthesitis, dactylitis and axial involvement, patients with PsA have psoriatic skin, such as epidermal hyperplasia, hyperkeratosis, parakeratosis, Munro's microabscesses and mixed dermal infiltrates. 2 In general, the severity of a patient's joint symptoms is consistent with the severity of the skin lesions. 3 In some cases, patients with PsA without psoriatic skin features only have arthritis damage or finger pain, which is similar to rheumatoid arthritis (RA). Generally, only about 13% of patients with PsA have rheumatoid factor in the blood and synovial fluid, while more than 80% of RA patients can be detected with rheumatoid factor. 4 The prevalence of PsA around the world is 0.3%-1%, 4 which differs in race and regions. In Western countries, the prevalence rates are about 0.02%, 0.42% and 0.25% in Sweden, Italy and the USA respectively, 5,6 whereas the rates are relatively lower in the Eastern countries, ranging from 0.01% to 0.1% in China, and <0.00001% in Japan. 6 PsA is not only harmful to an individual's health and life quality, but also may lead to a higher risk to suffer from cardiovascular events. 7 The pathogenesis of PsA is complex, and itmay be caused by genetic and environmental factors. Environmental aspects include infections (eg infectious diarrhea, human immunodeficiency virus infection), smoking, trauma and obesity. 8-11 In the researches for susceptibility genes for PsA, some human leukocyte antigen (HLA) genes are found as risk factors, and some HLA genes are found as protect factors. For instances, HLA-B*38 and HLA-Cw6 alleles are