PDCD1 and colorectal cancer: Further analysis of the TCR-β profiles of both PD-1+ and PD-1− populations in CD8+ T cells revealed that there was hardly any overlap in the TCR profiles of PD-1+ and PD-1−CD8+ T cell subpopulations in at least two human tumor tissues, colorectal cancer (CRC) and breast cancer, thus, it can be concluded that the intrinsic properties of the PD-1+ population determine its ability to recognize tumor antigens specifically (23).