Activation of the PI3K/AKT pathway is a common event in human cancers and is responsible for key aspects of the transformed phenotype.This pathway is activated in most breast cancers, but by different mechanisms including HER2 amplification, PI3K mutation and PTEN inactivation.Inhibition of this pathway is thus thought to be clinically useful, but the functional output of PI3K/AKT signaling and the corresponding clinical effects of its inhibition may be due to the mechanisms by which it is activated.The goal of this study was to determine the role of AKT activation in breast cancer as a function of mechanism of activation and whether inhibition of AKT signaling is a feasible approach to therapy.A selective allosteric inhibitor ofAKT kinase (AKTi) was used to interrogate a panel of breast cancer cell lines characterized for genetic lesions that activate PI3K/AKT signaling: HER2 amplification or PI3K or PTEN mutations in order to determine the biochemical and biologic consequences of inhibition of this pathway.