Although evidence of PD-1 expression in tumor cells has been provided at the mRNA (qRT-PCR and RNA-sequencing), and protein (IHC, immunofluorescence, western blotting, and FACS) levels in previous studies,21 22 127 131 133 we re-evaluated the expression of PDCD1 using the Cancer Cell Line Encyclopedia (Broad, 2019) in lung cancer (NSCLC and SCLC), colorectal, breast, melanoma, kidney, stomach and liver cancer cell lines. Here, PDCD1 is linked to melanoma.