They make up to 1–10% of peripheral T cells and up to 50% of hepatic T cells.1 They are also present at mucosal surfaces2 and have been shown to play important roles in settings of bacterial3 and viral infection4 as well as cancer.5 They express a semi‐invariant T cell receptor (Vα7.2) on their cellular surface,6 through which they recognize ligands presented by the nonclassical MHC class Ib molecule MR‐1 (MHC‐class I related protein 1).2 MR‐1 binds metabolites of the riboflavin metabolism pathway, found in certain bacteria such as Escherichia coli. Here, MR1 is linked to cancer.