In advanced melanoma patients, the group with the most (>50% cells) and complete loss of major histocompatibility complex(MHC) I expression on tumor cells showed primary resistance to anti cytotoxic T lymphocyte antigen 4 (CTLA-4) therapy, while MHC II expression positive (>1%) on tumor cells suggested favorable response to anti PD-1 therapy 137. The gene discussed is PDCD1; the disease is melanoma.