PD-L1 is expressed on a variety of cells, both hematopoietic and non-hematopoietic, whereas PD-L2 is selectively induced on fewer cells post-activation, especially APCs, and has a higher affinity to PD1 than PD-L1; however, recent studies have shown that PD-L2 can be found also on other immune cells and even on tumor cells under microenvironment stimulation [66]. This evidence concerns the gene PDCD1 and neoplasm.