Ankylosing spondylitis (AS), psoriasis (Ps) and psoriatic arthritis (PsA) are a group of autoimmune diseases which share some clinical features and some overlapping susceptibility loci such as endoplasmic reticulum aminopeptidase 1 (ERAP1) (formerly known as ARTS1) [2,3] and the interleukin 23 receptor (IL23R) [2,4,5]. This evidence concerns the gene ERAP1 and psoriatic arthritis.