STAT3 and cancer: Several mechanisms have been implicated in the regulation of PD-L1 expression by cancer cells such as activation of mitogenic and pro-survival pathways including MAPK and PI3K/AKT, increased activity of transcriptional factors HIF-1, STAT-3 and NF-κB, and presence of epigenetic modulators including miR-513, miR-570, miR-34a, miR-200 and miR-197 [14].