ENTPD1 and neoplasm: On the other hand, several recent studies have unveiled that not all intratumoral CD8+ T cells are equally tumor-reactive: the tumor-reactive CD8+ T cells are the most clonally expanded subpopulations with a tissue-resident memory phenotype and exhausted phenotype marked by CD103 and CD39, respectively; whereas the bystander CD8+ T cells are less clonally expanded and negative for CD103 and CD39 (Figure 1; Duhen et al., 2018; Simoni et al., 2018; Li H. et al., 2019; Scheper et al., 2019).