MAPK8 and neoplasm: Moreover, Massi et al. studied the intracellular pathways in stable PD-L1+ and PD-L1− subpopulations of a tumor cell line, and found that the PD-L1+ cells showed a constitutively higher degree of phosphorylation of ERK1/2, p38 and JNK, compared to that observed in PD-L1− cells12.