The relative sensitivity of 12 hormone receptor–positive breast cancer cell lines to the PI3K inhibitor LY294002. LY294002 was applied to the panel of hormone receptor–positive breast cancer cell lines and the concentration causing 50% growth inhibition (GI50) was determined and presented as relative sensitivity (−log GI50). The cell lines are presented in order of increasing PTEN protein expression (lane A) as determined using RPPA. PTEN-low (P = 0.0007) and PTEN-mutant (P = 0.02) cell lines are significantly more sensitive to growth inhibition by LY294002 than PIK3CA-mutant cell lines. Lane B, AKT phosphorylation at Ser473 determined using RPPA [color coded as in lane A (i.e., green, low; black, mean; and red, high AKT phosphorylation)]. Lane C, cell line mutation status.