MR1 and cancer: Immune escape via defects or downregulation of MHC-I molecules is one of the main mechanisms cancer cells use to prevent elimination by cytotoxic or natural killer cells.17 Similarly, overexpression of nonclassical MHC-I also helps avoid targeting by cytolytic cells.18 MR1 is a nonclassical MHC class I-like molecule responsible for bacterial and fungal homeostasis in the mucosa.