PDCD1 and neoplasm: Our early studies using PD-1-deficient mice and blocking antibodies revealed the immunoinhibitory role of PD-1/PD-L1 signaling in anti-tumor and anti-viral immunity (Freeman et al., 2000; Iwai et al., 2002; Iwai et al., 2003; Iwai et al., 2005; Okazaki et al., 2013).