This discovery, along with an abundance in humans3,4, conservation across species5–7, and the defective immune response of MR1/MAIT cell-deficient mice to infection by Francisella tularensis8, Klebsiella pneumoniae9, and Mycobacterium tuberculosis10, has led to the notion that these cells have an important function in controlling bacterial infections. The gene discussed is MR1; the disease is infection.