Epidermal growth factor EGF stimulation of mammary epithelial cells MECs inhibits differentiation and apoptosis. Excess activation of signaling pathways downstream of the EGF receptor has been directly linked to breast cancer development and chemotherapeutic resistance. Therefore, the dissection of these pathways in normal MECs is critical to the understanding of signaling events that impact breast cancer outcome. HC11 mouse MECs differentiate in response to lactogenic hormone resulting in expression of milk proteins including beta-casein. Previous studies have shown that EGF blocks differentiation through activation of the RasMekErk and the PI-3-K pathway. Therefore, specific chemical inhibitors of signal transduction pathways, activated Ras RasV12, dominant negative-Ras DN-RasN17, dominant negative-Akt adenovirusDN-Akt-adeno and a conditionally active-Akt CA-Akt were used to analyze the role of the Ras and PI-3-K pathway in blocking HC11 lactogenic differentiation.