PDCD1 and neoplasm: However, the function of PD-1 in some suppressive immune cells and tumor cells has been controversial (Kleffel et al., 2015; Lowther et al., 2016; Stathopoulou et al., 2018; Kim H. R. et al., 2019; Kamada et al., 2019; Wang X. et al., 2020; Kumagai et al., 2020; Yoshida et al., 2020; Lim et al., 2021).