Estrogen-related receptor alpha (ERR alpha), a NR3 Steroid Receptor, was isolated based on sequence similarity in its DNA-binding domain to estrogen receptor alpha (ER alpha). ERR alpha has been shown to regulate the promoters of lactoferrin, medium-chain acyl CoA dehydrogenase, osteopontin, and thyroid receptor alpha, and it may affect cellular energy balance and bone formation. ERR alpha binds as a monodimer to the extended half-site TNAAGGTCA and as a homodimer to the estrogen response element (ERE) and is a constitutive activator of the estrogen response element and the palindromic thyroid hormone response element (TRE(pal)) but not of the glucocorticoid response element (GRE). ERR alpha 1 is the major isoform expressed in human breast cancer cell lines. Recent studies have shown that Phe-329 is responsible for the constitutive activity of ERR alpha. ERR alpha is a potential biomarker for unfavorable clinical outcome and, possibly, hormonal insensitivity in breast tumors. ERR alpha status may be predictive of sensitivity to hormonal blockade therapy, and ERR alpha status may also be predictive of ErbB2-based therapy such as Herceptin. Moreover, ERR alpha may be a candidate target for therapeutic development. ERRalpha null mice have altered regulation of genes involved in adipogenesis In mouse, ERR alpha is expressed in many adult and embryonic tissues (particularly at the onset of ossification) as well as in several osteoblast cell lines. ERR alpha expression has been documented in mouse in brain, spinal cord, pituitary gland, heart, intestine, bone, brown adipose tissue, heart, uterus, cervix, nerve, skeletal muscle, and vagina. ESTs have been isolated from human tissue libraries, including cancerous blood, brain, breast, cervix, colon, duodenum, eye, head/neck, kidney, liver, lung, ovary, pancreas, skeletal muscle, skin, stomach, and uterus, and normal adrenal, blood, brain, colon, embryo, eye, head/neck, heart, kidney, prostate, skeletal muscle, testis, and uterus.