Given that MAIT cells specifically recognize riboflavin metabolites as antigens presented by MR1 molecules, it is of particular interest that riboflavin-producing bacterial species have been found to be present at higher relative abundance in women with breast cancer, including Enterobacteriaceae (the family that includes E. coli), Bacillus, and Staphylococcus species [5, 6]. Here, MR1 is linked to breast cancer.