Finally, the notorious difference in the genetic association of ERAP1 and ERAP2 with disease, namely the epistasis of ERAP1 and the susceptibility MHC-I allele in AS, psoriasis and BD, the non epistatic association of ERAP2 in the two former diseases, and, as far as it is known, the lack of association of ERAP2 with BD, may be better explained by the separate activities of both enzymes on the MHC-I peptidomes, than by assuming a strong influence of ERAP2 on ERAP1 activity. This evidence concerns the gene ERAP1 and psoriasis.