Table 3. Affinity (K_d) of identified estrogen receptors as a function of their cellular localization and the system in which they have been measured.

Note that GPR30 was identified in the plasma membrane in the studies of Thomas and colleagues [254] but in the endoplasmic reticulum membrane in the studies of Revankar, Bologa, Prossnitz and colleagues [44,209]. Thomas et al. (2005) found Bmax values of 114 pM and 100 pM for SKBR3 cells and HEK-293 cells transfected with GPR30 [254].

	Nuclear		Membrane
	Kd	Origin	Kd	Origin
ERα	0.1 nM	in vitro synthesized human ERα [139]	0.30 nM	ERα-GFP–transfected COS7 cells [44]
	0.283 nM	ERα–transfected CHO cells [205]	0.287 nM	CHO cells [205]
ERβ	0.6 nM	Clone 29, rat prostate cDNA library [140]	0.38 nM	ERβ-GFP–transfected COS7 cells [44]
	0.6 nM	In vitro synthesized human ERβ [139]	1.17 nM	CHO cells [205]
	1.23 nM	ERβ–transfected CHO cells [205]
ER-X			1.6 nM
GPR30			5.7 nM	GPR30-GFP–transfected COS7 cells [44]
			6.6 nM	GPR30-GFP–transfected COS7 cells [209]
			2.7 nM	ERα-/ERβ-/GPR30+-transfected SKBR3 cells [254]
			3.3 nM	ERα-/ERβ-/GPR30+-transfected (HEK)293 cells [254]
E2-BSA				Membrane fraction:
			4.5 nM	Hypothalamus [204]
			8.7 nM	Olfactory bulbs [204]
			30.2 nM	Cerebellum [204]

Abbreviations: ERα, estrogen receptor α ; ERβ, estrogen receptor β ; GFP , green fluorescent protein ; GPR30, G Protein Receptor 30 ; (HEK)293 cells, Human Embryonic Kidney cells; CHO cells; Chinese Hamster Ovary cells; COS7 cells, Monkey Kidney fibroblast; SKBR3 cells, Human Breast Cancer cells.