Previously, we established a phosphorylation code (P7-score) for ERα by measuring phosphorylation at 7 different sites on ERα in breast tumours. P7-score is a more accurate prognostic marker than ERα alone. We predict that cell signaling pathways are different between high and low P7-score tumours. Preliminary data, based on high through-put antibody array screens using extracts from high and low-P7 tumours and immunohistochemistry (IHC), identified differential expression of several kinases. One was mTOR but contrary to earlier reports, higher phosphorylated-mTOR, measured by IHC, associated with better clinical outcome and low-P7 scores in this cohort of ER+ patients subsequently tamoxifen treated. Since the relationship between ERα and mTOR is not well understood and motif analysis suggested that several serines in ERα were potential FRAP/mTOR substrates, the hypothesis that mTOR directly interacts and phosphorylates ERα was examined to gain greater insight into the role played by mTOR in ERα signaling. In vitro kinase assays and mass spectrometry, suggested that recombinant mTOR could phosphorylate ERα. Under endogenous conditions, using MCF7 ER+ breast cancer cells ERα co-immunoprecipitated (co-IP) with mTOR and this interaction was increased in the presence of estrogen. mTOR signals through two complexes, mTORC1 and mTORC2. ERα/mTOR interaction was only partially rapamycin (inhibiting mTORC1 selectively) sensitive, suggesting ERα may bind mTOR directly, or through mTORC2. We carried out proximity-ligation-assays (PLA) to investigate endogenous ERα/mTOR interactions in situ, in intact cells. Consistent with co-IP, PLA results support protein-protein interactions between ERα and mTOR, and demonstrated both nuclear and cytoplasmic complexes. These data support the novel hypothesis that ERα is an mTOR substrate. The results presented support a direct role for mTOR in ERα phosphorylation. How this impacts mitogenic and survival functions of estrogen signalling and/or endocrine resistance in breast cancer remains unclear, but would be expected to influence treatment combinations and scheduling.