AS belongs to the so-called “MHC-I-opathies”, together with the psoriasis, Behçet’s disease, birdshot uveitis and acute anterior uveitis for which GWAS have highlighted the association to ERAP1 and, sometimes, ERAP2, apart from the involvement of a key specific HLA-class I gene [47]. The gene discussed is ERAP2; the disease is psoriasis.