PDCD1 and neoplasm: TIGIT and PD-1 may also be co-expressed by CD103+ CD8+ resident memory T cells (Trm), a T cell subpopulation that mediates anti-tumor immunity that may be derived from stem cell-like memory T (Tscm) or effector T (Teff) cells (Corgnac et al., 2020; Craig et al., 2020; Park et al., 2019).