Among them, a small cell subset expressing PD-1 and the transcription factor TCF1 (encoded by Tcf7) can respond to anti-PD-1 immunotherapy and differentiate into highly cytotoxic, terminally exhausted TILs that mediate long-term tumor control (Im et al., 2016, Miller et al., 2019, Siddiqui et al., 2019). The gene discussed is PDCD1; the disease is neoplasm.