Current models posit that T cells exposed to chronic high levels of antigen, as in unresolved infection and tumor, contain populations of “precursor exhausted” cells (Tpex: PD1+, TCF1- and TIM-3-) and “terminally exhausted” T cells (Tex: PD-1+, TCF1-, and TIM-3+) (25). The gene discussed is PDCD1; the disease is infection.