ERAP1 and Behcet disease: In particular, the epistatic interactions observed between ERAP1 and HLA-B27 in AS, HLA-Cw6 in Ps and HLA-B51 in BD, strengthen the central role of antigen presentation in the so-called “MHC-I-opathy”, although the allelic variants of ERAPs specifically associated with each disease may be different30.