IL23A and psoriasis: Thus, in addition to HLA-C, three pathways of psoriasis susceptibility were identified: TH17 pathway [IL-12B (encoding the p40 subunit of IL-23 and IL-12), IL-23A (encoding the p19 subunit of IL-23), IL-23R (encoding a subunit of the IL-23 receptor)], NFκB pathway (TNFAIP3, TNIP1), and TH2 pathway (IL-4, IL-13).