ESR1 and breast cancer: PTHrP secretion, which we previously demonstrated to be E2 inducible via ERα in MCF-7 cells [12] and is TGFβ inducible via Smad signaling in some bone-tropic ER- breast cancer cell lines [13,14,15,43], was also induced by TGFβ in bone-tropic MCF-7 cells, demonstrating additive effects when combined with E2 (Figure 5A, left panel).