The function of MR1 (major histocompatibility complex, class I-related, ENSG00000153029) is as follows. Antigen-presenting molecule specialized in displaying microbial pyrimidine-based metabolites to alpha-beta T cell receptors (TCR) on innate-type mucosal-associated invariant T (MAIT) cells (PubMed:19416870, PubMed:23457030, PubMed:22692454, PubMed:23051753, PubMed:24101382, PubMed:23846752, PubMed:26795251). In complex with B2M preferentially presents riboflavin-derived metabolites to semi- invariant TRAV1.2 TCRs on MAIT cells, guiding immune surveillance of the microbial metabolome at mucosal epithelial barriers (PubMed:20581831, PubMed:24101382, PubMed:24695216, PubMed:26795251). Signature pyrimidine-based microbial antigens are generated via non- enzymatic condensation of metabolite intermediates of the riboflavin pathway with by-products arising from other metabolic pathways such as glycolysis. Typical potent antigenic metabolites are 5-(2- oxoethylideneamino)-6-D-ribitylaminouracil (5-OE-RU) and 5-(2- oxopropylideneamino)-6-D-ribitylaminouracil (5-OP-RU), products of condensation of 5-amino-6-D-ribityaminouracil (5-A-RU) with glyoxal or methylglyoxal by-products, respectively (PubMed:24695216, PubMed:32958637, PubMed:32709702). May present microbial antigens to various TRAV1-2-negative MAIT cell subsets, providing for unique recognition of diverse microbes, including pathogens that do not synthesize riboflavin (PubMed:27527800, PubMed:31113973). Upon antigen recognition, elicits rapid innate-type MAIT cell activation to eliminate pathogenic microbes by directly killing infected cells (PubMed:23846752, PubMed:24695216, PubMed:27527800). During T cell development, drives thymic selection and post-thymic terminal differentiation of MAIT cells in a process dependent on commensal microflora (By similarity). Acts as an immune sensor of cancer cell metabolome (PubMed:31959982). May present a tumor-specific or -associated metabolite essential for cancer cell survival to a 'pan- cancer' TCR consisting of TRAV38.2-DV8*TRAJ31 alpha chain paired with a TRBV25.1*TRBJ2.3 beta chain on a non-MAIT CD8-positive T cell clone (MC.7.G5), triggering T cell-mediated killing of a wide range of cancer cell types (PubMed:31959982). {ECO:0000250|UniProtKB:Q8HWB0, ECO:0000269|PubMed:19416870, ECO:0000269|PubMed:20581831, ECO:0000269|PubMed:22692454, ECO:0000269|PubMed:23051753, ECO:0000269|PubMed:23846752, ECO:0000269|PubMed:24101382, ECO:0000269|PubMed:24695216, ECO:0000269|PubMed:26795251, ECO:0000269|PubMed:27527800, ECO:0000269|PubMed:31113973, ECO:0000269|PubMed:31959982, ECO:0000269|PubMed:32709702, ECO:0000269|PubMed:32958637}. Allele MR1*01: Presents microbial-derived metabolite 5-OP-RU to semi-invariant TRAV1.2-TRAJ33-TRBV6.1 (A-F7) TCR on MAIT cells (PubMed:39589872). Presents nucleobase carbonyl adducts generated during oxidative stress. Captures M3Ade, a nucleobase adduct composed of one adenine modified by a malondialdehyde trimer, for recognition by MR1-restricted T cell clones expressing a polyclonal TCR repertoire (PubMed:39701104). Displays moderate binding affinity toward tumor- enriched pyridoxal and pyridoxal 5'-phosphate antigens (PubMed:39589872). {ECO:0000269|PubMed:39589872, ECO:0000269|PubMed:39701104}. Allele MR1*04: Presents tumor-enriched metabolite pyridoxal to pan-cancer 7.G5 TCR on T cells enabling preferential recognition of cancer cells. May act as an alloantigen. {ECO:0000269|PubMed:39445059, ECO:0000269|PubMed:39589872}.