HLA-C and psoriasis: HLA-C and IL-23R were more strongly associated with psoriasis alone, and IL-12B with PsA.[44] A smaller GWAS identified a novel PsA (and potentially psoriasis) locus on chromosome 4q27 that harbors the interleukin 2 (IL-2) and interleukin 21 (IL-21) genes.[49]