Several phosphoinositide 3-kinase (PI3K) catalytic subunit inhibitors are currently in clinical trial. We therefore sought to examine relationships between pharmacologic inhibition and somatic mutations in PI3K catalytic subunits in estrogen receptor (ER)-positive breast cancer, in which these mutations are particularly common. RNA interference (RNAi) was used to determine the effect of selective inhibition of PI3K catalytic subunits, p110αand p110β, in ER + breast cancer cells harboring either mutation (PIK3CA) or gene amplification (PIK3CB). p110αRNAi inhibited growth and promoted apoptosis in all tested ER + breast cancer cells under estrogen deprived-conditions, whereas p110βRNAi only affected cells harboring PIK3CB amplification. Moreover, dual p110α/p110βinhibition potentiated these effects. In addition, treatment with the clinical-grade PI3K catalytic subunit inhibitor BEZ235 also promoted apoptosis in ER + breast cancer cells. Importantly, estradiol suppressed apoptosis induced by both gene knockdowns and BEZ235 treatment. Our results suggest that PI3K inhibitors should target both p110αand p110βcatalytic subunits, whether wild-type or mutant, and be combined with endocrine therapy for maximal efficacy when treating ER + breast cancer.