CD28 and neoplasm: The other master co-inhibitory receptor is the programmed cell death protein 1, PD-1, which is also a CD28 family member and binds to two ligands PD-L1 (B7-H1, CD274) as well as PD-L2 (B7-DC, CD273), expressed at the surface of other immune cells as well as non-lymphoid tissues, including tumor cells [90].