As in the case of PsO and IBD, predisposition to AS and PsA is linked to genetic variations in the IL12B region, which encodes the IL-12 p40 component shared by IL-12 and IL-23; PsA and PsO are also linked to a variation in the IL23A region that encodes the IL-23 p19 subunit. This evidence concerns the gene IL23A and inflammatory bowel disease.