PDCD1 and cancer: PD-L1 together with PD-L2 are binding and functional partners of PD-1, expressed on the surface of many organ cells and in various tissues (Freeman et al., 2000; Latchman et al., 2001) playing a dominant role in the suppression of T cell responses, especially in the tumor microenvironment (Zou and Chen, 2008), thereby preventing effector immune cells from killing cancer cells (Azuma et al., 2008).