Although most researchers have focused on PD-1 evaluation on TILs, recent data have shown that PD-1 can also be expressed in TCs by a secondary mechanism, as identified in non-small cell lung carcinoma (NSCLC), colon carcinoma, melanoma, hepatocellular carcinoma, pancreatic adenocarcinoma, and triple-negative mammary carcinoma [36]. This evidence concerns the gene PDCD1 and melanoma.