CD274 and familial pancreatic carcinoma: In MCF-7 and BT-549 breast tumor spheres, the repressive histones, H3K9me3 and H3K27me3 bind weakly to the PD-L1 promoter, while the positive regulatory histone, H3K4me3 binds strongly; and PD-L1 expression is mediated by histone methylation.101 Furthermore, PD-L1 expression can be initiated by the MLL1-H3K4me3 axis in pancreatic cancer102; H3K4me3 is enriched in the PD-L1 proximal promoter in pancreatic cancer cells in vitro, while MLL1 is an H3K4 methylation-specific histone methyltransferase.