PDCD1 and melanoma: Furthermore, animal models showed PD-1 blockade improves effector T-cell infiltration of tumors and response to CTLA-4 blockade against melanoma and colorectal tumors [12, 13], especially so with concurrent blockade, and that PD-L1 overexpression can be used by melanoma tumors as an escape mechanism to CTLA-4 blockade [14].