PsA is a polygenic immune-mediated disease: genome-wide association studies (GWAS) have identified many genes/genomic loci increasing susceptibility for PsA, many of which are also common to psoriasis uncomplicated by arthritis; these include HLA-A, HLA-B, HLA-C, IL23R, CSF2 (Colony Stimulating Factor 2 or granulocyte-macrophage colony stimulating factor), TRAF3IP2 (TRAF3 Interacting Protein 2), NOS2 (Nitric Oxide Synthase 2) (31, 32). This evidence concerns the gene NOS2 and arthritic joint disease.