PDCD1 and melanoma: Kleffel et al. (2015) [4], Li et al. (2017) [5], Du et al. (2018) [6], Pu et al. (2019) [7], and Wang et al. (2020) [8] reported that melanoma, hepatocellular carcinoma, pancreatic ductal adenocarcinoma, and lung cancer cells expressed PD-1, respectively.