Previous studies have shown that BTLA and PD-1 are co-expressed in human melanoma CD8+ T cells and hepatocellular carcinoma CD4+ T cells (22, 23), and both BTLA and PD-1 can recruit SH2-containing protein tyrosine phosphatase (SHP)-1 and SHP-2 to suppress T-cell receptor signaling (24, 25). Here, PDCD1 is linked to melanoma.