PDCD1 and colorectal carcinoma: Nonetheless, due to PD-1 signaling is always mediated via engagements of PD-L1 or PD-L2 ligands and tumor environments are complex, the mechanism by which tumor cell-intrinsic PD-1 promotes CRC tumor growth is not only limited to the AKT and ERK signals, more about the principles among this process need to be further investigated.