On the opposite, low active forms of ERAP1 are involved in the onset of BSCR and BD thus subdividing these “MHC-I-opathies” in two categories where, in any case, ERAP1 plays a pivotal role in the occurrence of the “mis-immunopeptidome” with a pathogenetic relevance (Figure 1). The gene discussed is ERAP1; the disease is Behcet disease.