CD274 and breast cancer: Pre-clinical in vitro studies indicated that PDL1 expression was reduced in ERα+ (MCF-7, T47D, CAMA-1, ZR-75-1, and BT-474) compared to ERα− (MDA-MB-231, HCC1937, and BT-549) BC cell lines, suggesting that ER signaling may lead to the downregulation of PDL1 (Figure 1).