MR1 and cancer: MR1 is a non-polymorphic MHC class I-like molecule encoded in human chromosome 1 with many similarities to canonical class I molecules, but with the added distinction of being located mainly in the endoplasmic reticulum and endosomal vesicles.11,12 MR1 presents microbial-derived vitamin B metabolites to an innate T cell called mucosal-associated invariant T cells13,14 resulting in their proliferation and secretion of pro-inflammatory cytokines to control infection.3 However, the exact involvement of the MR1 in cancer immunology is unknown.