STAT1 and neoplasm: This is in line with recent publications suggesting that the main role in the regulation of PD-L1 expression is played by STAT1. 52In tumor cell lines from several cancer types, small interfering RNA (siRNA) knockdown of STAT3 did not reduce IFN-γ- or interleukin-27-induced PD-L1 protein expression, while siRNA knockdown of STAT1 did.53