ESR1 and breast carcinoma: Furthermore, PD-L1 (CD274) protein is expressed at low levels in ER+/HER2−/TWIST1− epithelial breast cancer cell lines, including MCF7, ZR-75-1, and T47D, while it is at high levels in ER−/HER2−/TWIST1+ TNBC cell lines, including MDA-MB-436, BT549 and SUM1315 (Figure 1E).