An emerging mechanism of endocrine resistance in aberrant signaling through the phosphatidylinositol 3-kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) signaling pathway7-9. Growing evidence supports a close interaction between the mTOR pathway and ER signaling. A substrate of mTOR complex 1 (mTORC1), called S6 kinase 1, phosphorylates the activation function domain 1 of ER, which is responsible for ligand-independent receptor activation10. Everolimus is a sirolimus derivative that inhibits mTOR through allosteric binding to mTORC111. In preclinical models, the use of everolimus in combination with aromatase inhibitors results in synergistic inhibition of the proliferation and induction of apoptosis12. In a randomized, phase 2 study comparing neoadjuvant everolimus plus letrozole with letrozole alone in patients with newly diagnosed ER-positive breast cancer, the response rate for the combination was higher than that for letrozole alone13. Recently, the Breast Cancer Trials of Oral Everolimus-2 (BOLERO-2) study showed that the addition of everolimus to exemestane significantly improved progression-free survival, with observed medians of 6.9 and 2.8 months, corresponding to a 57% reduction in the hazard ratio14.