CD274 and neoplasm: PD1 binds to its PD-L1 ligand, which is present on tumor cells and antigen-presenting cells, causing a cascade of intracellular reactions leading to inactivation of the CD28 protein and inhibition of T cell activation (Walunas et al., 1996) (Brown et al., 2003) (Ishida et al., 2002) (Freeman et al., 2000).