A great range of immune checkpoint molecules, such as PD-1 (Okazaki and Honjo, 2007), PD-L1 (Herbst et al., 2014), CD73 (Allard et al., 2017), Lag-3 (Andrews et al., 2017), B7-H3 (Suh et al., 2003), TIM-3 (Anderson et al., 2016), CTLA-4 (Krummel and Allison, 1995), TIGIT (Kurtulus et al., 2015), and VISTA (Wang et al., 2011) in the tumor microenvironment (TME) are involved in important mechanisms that prevent effector T cells’ anti-tumor activities. The gene discussed is PDCD1; the disease is neoplasm.