PDCD1 and neoplasm: sPD-1, encoded by the PDCD1 gene, is thought to be primarily generated by alternative mRNA splicing; sPD-L1 is also thought to be produced by alternative splicing of the PD-L1 mRNA as well as by proteolytic cleavage of the extracellular fraction of membrane-bound PD-L1 expressed on tumor cells or mature dendritic cells; some sPD-L1 isoforms are also believed to originate from proteolysis of PD-L1 expressed on the surface of secreted cellular exosomes [42,43,44].