The effector T cell response appears to be sustained by a “stem-like” population of TCF-1+ PD-1+ CD8 T cells that are required for tumor control in response to immunotherapy (Brummelman et al., 2018; He et al., 2016; Im et al., 2016; Jansen et al., 2019; Kurtulus et al., 2019; Sade-Feldman et al., 2018; Siddiqui et al., 2019; Utzschneider et al., 2016). This evidence concerns the gene PDCD1 and neoplasm.