Both psoriasis and PsA have a substantial genetic component that influences an individual’s susceptibility; indeed there are now 63  confirmed risk loci for psoriasis in populations of European orign.5 The identification of risk loci that are specific for the development of PsA in patients with psoriasis has been more challenging, but evidence is now emerging of loci associated at genome-wide significance thresholds with PsA and not PsC, including loci at 5q31, IL23R, PTPN22, TNFAIP3 and HLA-B. Here, PTPN22 is linked to psoriasis.