Actually, we demonstrate here that the common underlying molecular mechanism of HLA class-I loss observed in the MCC cell lines investigated by us is epigenetic silencing of the antigen presentation machinery (APM): The transporter subunits TAP1 and TAP2 as well as the immunoproteasome subunits LMP2 and LMP7 are simultaneously down-regulated by histone hypoacetylation. This evidence concerns the gene PSMB9 and Merkel cell skin cancer.