We examined the capacity of several growth factors to convey RTK-mediated acquired resistance to BYL719 (alpelisib), a PIK3CA-specific inhibitor that has recently entered the clinic [44], in two luminal breast cancer cell lines with PIK3CA mutations, MCF7 and T47D, and in MDA-MB-453, a PI3K mutant HER2 + breast cancer cell line (Table S1). Here, PIK3CA is linked to breast cancer.