PIK3CD and breast cancer: In the same vein, the work by Costa et al. [67] showed that PI3Kα inhibition by BYL719 initially abrogated PI3K signalling, but within 24 h induced a rebound in PI3K activation (indicated by the elevated phosphoinositide PIP3 level) in HER2+ or PIK3CA-mutant luminal BC cells[67].