PDCD1 and neoplasm: The expression of PD-1 and PD-L1 on tumor cells, freshly collected in vivo, was found to be much higher than on cells cultured in vitro, which, due to the influence of cytokines, may suggest that it is the tumor microenvironment that has a large impact on shaping the expression of elements of the PD-1/PD-L1 pathway [9,22,23,24].