PDCD1 and cancer: The programmed cell death protein 1 (PDCD1) gene encodes an immune-inhibitory receptor expressed by activated T cells that engages with programmed death ligand 1 (PD-L1) on tumor cells to allow cancers to escape a normal T-cell mediated apoptosis checkpoint and proliferate.41 Higher levels of PD-L1 protein and mRNA have been seen in functional and aggressive PitNETs compared to NF-PitNETs and non-aggressive tumors.42PDCD1 methylation levels are lower in aggressive PitNETs, making these tumors more capable of bypassing the immune checkpoint.15