PDCD1 and neoplasm: Certain cell-surface markers of ongoing and chronic activation such as PD-1, CD39, CD69, CD103, or CD137 may also help to delineate T cell subpopulations (typically CD8+) that are enriched for clonally expanded tumor-reactive T cells (Gros et al., 2014; Duhen et al., 2018; Murray et al., 2016), therefore culturing selected TIL subsets, such as PD-1+ T cells (Inozume et al., 2010), is a feasible option.