Akt, when activated by IGF/insulin, can phosphorylate forkhead transcription factors. We undertook this study to determine whether epidermal growth factor (EGF) treatment could produce a signaling cascade resulting in phosphorylation of the forkhead transcription factor FKHR in a breast cancer cell line, MDA-MB-231. After establishing ErbB1, cbl, PI3 kinase and Akt were activated in EGF treated MDA-MB-231, we determined by immunoblot with FKHR antiserum that the electrophoretic mobility of FKHR was retarded after EGF treatment. This mobility retardation was reversible by treatment with alkaline phosphatase, and immunoblot with phospho-Ser 256 FKHR antibody further con®rmed phosphorylation on an Akt consensus site after EGF treatment. EGF stimulated FKHR phosphorylation was blocked by the PI3 kinase inhibitor LY294002, and the ErbB1 inhibitor AG1478. FKHR immunoblotting after puri®cation of nuclear and cytoplasmic proteins showed that EGF induced a simultaneous increase of FKHR in the cytoplasm and decrease in the nucleus. This ®nding was con®rmed by immuno¯uorescence staining. Treatment of cells with pharmacological inhibitors of PI3 kinase or ErbB1 blocked this e ect. Thus, these results demonstrate the phosphorylation and nuclear exclusion of FKHR after EGF treatment by a PI3 kinase dependent mechanism, and represent the ®rst report of growth factor regulation of endogenous FKHR localization Oncogene (2000) 19, 4574 ± 4581.