Complete suppression of mTORC1 by RMC-6272 causes apoptosis in ER+/HER2− breast cancer cell lines, particularly in those that harbor mutations in PIK3CA or PTEN, due to inhibition of the rapamycin resistant, mTORC1 substrate 4EBP1 and reduction of the pro-survival protein MCL1. This evidence concerns the gene PIK3CA and breast cancer.