Psoriasis and psoriatic arthritis are immune disorders with a complex polygenic basis. HLA-Cw6, which lies in the major histocompatibility region on chromosome 6, is considered the major genetic determinant of psoriasis. Recent genome-wide association studies have identified new variants outside of the MHC with relevance to the immunology of psoriasis. Variants in or near genes that encode subunits of cytokines (IL12B, IL23A) or cytokine receptors (IL23R) are interesting given that the gene product of IL12B, p40, is the target of a recently approved monoclonal antibody therapy for psoriasis (ustekinumab). Association with psoriasis and psoriatic arthritis has been found in TNFAIP3 and TNFIP1, ubiquitin ligases in the NF-κB pathway, and IL13, a Th2 cytokine. Copy number variation of human beta-defensin and late cornified envelope genes also associate with psoriasis. Many of these genetic variations also associate with immune disorders considered psoriatic co-morbidities, including Crohn's disease and diabetes.