PRF1 and neoplasm: Although there are currently no reports of tumor antigens presented by MR1 to MAIT cells13–16, MAIT cells upregulate granzyme B and perforin following activation5,17,18 and have the capacity to kill tumor cells pulsed with the MAIT cell antigen 5-(2-oxopropylideneamino)-6-d-ribitylaminouracil (5-OP-RU)19, suggesting that MAIT cells are capable of eliciting direct anti-tumor immune responses.