The estrogen-related receptor ERR, belonging to the nuclear receptor superfamily, is the first orphan nuclear receptor to be identified. ERR comprise ERRα, ERRβ and ERRγ. ERRs participate in estrogen signal pathway in various patterns and they share common transcriptional target genes with estrogen receptor in muscles and breast. Analysis of ERR expression in human breast cancer has proposed ERRα and ERRγ as prognostic markers of this cancer. ERR also play an pivotal role in the metabolism pathway. The natural ligand of ERRs have not been identified till now, so identification of modulators (positive or negative) of ERR activities would be highly useful in understanding of estrogen-related pathologies, such as human osteoporosis, breast cancer and diabetes.