Key resources table.

Reagent type (species) or resource	Designation	Source or reference	Identifier	RRID
Recombinant DNA reagent	Estrogen Receptor Alpha Ligand Binding Domain WT or Y537S	Fanning SW, Mayne CG, Dharmarajan V, Carlson KE, Martin TA, Novick SJ, Toy W, Green B, Panchamukhi S, Katzenellenbogen BS, Tajkhorshid E, Griffin PR, Shen Y, Chandarlapaty S, Katzenellenbogen JA, Greene GL. Estrogen receptor alpha somatic mutations Y537S and D538G confer breast cancer endocrine resistance by stabilizing the activating function-2 binding conformation. eLife. 2016;5:e12792. doi: 10.7554/eLife.12792.
Strain, strain background (Protein Expression)	E.coli BL21(DE3)	https://www.neb.com/products/c2527-bl21de3-competent-e-coli#Product%20Information
Cell line (Mammary Cells)	MCF7, MCF7 HA-Y537S, MCF7 HA-D538G, T47D, CAMA-1, ZR75-1, MDA-MB-361	ATCC; this paper; this paper; ATCC; ATCC; ATCC; ATCC	ATCC HTB-22, ATCC HTB-23, ATCC HTB-21, ATCC CRL-1500, ATCC HTB-27	CVCL_0031, CVCL_0553, CVCL_1115, CVCL_0588, CVCL_0620
Antibody	anti-ERα antibody F10	Santa Cruz Biotechnology	sc-8002	AB_627558
Antibody	anti-actin antibody	Santa Cruz Biotechnology	sc-69879	AB_1119529
Antibody	anti-HA antibody	Cell Signaling	C29F4	AB_1549585
Software	CCP4i	https://www.ccp4.ac.uk/ccp4i_main.php		SCR_007255
Software	VMD	https://www.ks.uiuc.edu/Research/vmd/		SCR_004905
Software	Gaussion G09	http://gaussian.com/		SCR_014897
Software	GaussView	http://gaussian.com/		SCR_014897
Software	HDX Workbench	http://hdxworkbench.com/
Software	Bionavigator	PamGene
Commercial assay or kit	PamStation96	PamGene
Commercial assay or kit	MycoAlert Mycoplasma Detection Kit	Lonza	LT07-518
Chemical compound, drug	protease inhibitor cocktail solution III	CalBiochem	539134
Chemical compound, drug	4-hydroxytamoxifen	Tocris	3412
Chemical compound, drug	ICI 182–780 (fulvestrant)	Tocris	1047/1
Chemical compound, drug	Raloxifene	Sigma- Aldrich	R1402
Chemical compound, drug	Estradiol	Sigma- Aldrich	E8875