MTOR and breast cancer: In particular, the phosphatidylinositol 3' kinase (PI3K)/mTOR pathway has been linked in a variety of studies to acquired resistance to endocrine therapies both preclinically and clinically [4], and recent clinical results have validated this idea by showing that the mTOR inhibitor everolimus in combination with the aromatase inhibitor exemestane extends survival in patients with metastatic ER+ breast cancer who have progressed on prior endocrine therapy [5].