Tamoxifen (ICI 47699) is a selective estrogen receptor modulator (SERM) which blocks estrogen action in breast cells and can activate estrogen activity in other cells, such as bone, liver, and uterine cells^[1][2][3]. Tamoxifen is a potent Hsp90 activator and enhances the Hsp90 molecular chaperone ATPase activity. Tamoxifen activates autophagy and induces apoptosis^[4]. In Vitro: Tamoxifen (ICI 47699) shows strong inhibition of MCF-7 cells (EC₅₀=1.41 μM) and to a lesser extent the T47D cells (EC₅₀=2.5 μM) but does not affect the MDA-MB-231 cells^[2]. In Vivo: The Tamoxifen-inducible gene knockout strategy has clear advantages in that expression of a gene can be ablated in adult mice at will in a tissue specific manner. To study the role of Med1 in adult heart, 7-week old TmcsMed1^-/- mice are given a daily Iintraperitoneal injection of Tamoxifen at a dose of 65 mg/kg for 5 days and killed at selected intervals thereafter. qPCR analysis of RNA shows that the Med1 expression begin to decrease after 3 days of Tamoxifen injection (about 70% decrease), and by 5 days of injection, Med1 expression is almost non-detectable in the heart. Tamoxifen-inducible cardiac-specific disruption of Med1 (TmcsMed1^-/-) in adult mice causes dilated cardiomyopathy^[3].