CDK4 and breast cancer: These pathways include PI3K/Akt/mTOR pathway3–5, cyclin D1/CDK4/6 pathway6–10, fibroblast growth factor receptor pathway11,12, IGF-1 pathway13, etc. Inhibitors of mTOR and CDK4/6 have already been approved by FDA to be used to treat endocrine-resistant breast cancers.