PDCD1 and cancer: Beyond classical NK cell receptors (KIRs, LIRs, and NKG2A), numerous other immune checkpoints contribute to NK cell dysfunction in various cancers and chronic infections, including CTLA-4, PD-1, B7-H3, as well as LAG-3, TIGIT & CD96, TIM-3, checkpoint members of the Siglecs family (Siglec-7/9), CD200, and CD47 [46].