ESR1 and breast cancer: Similarly, in the ER-/HER2+ BC cell lines HCC1954 and UACC893, both of which contain PIK3CA mutations, combination treatment with PI3K inhibitors alpelisib or GDC-0077 (inavolisib) plus trastuzumab induced anti-proliferative effects and prominent apoptosis compared to single-drug treatments [186].