The germ cell nuclear factor (GCNF), which is also known as RTR (retinoid receptor-related testis-associated receptor) is a member of the nuclear receptor superfamily. As a natural ligand remains to be discovered, GCNF is referred to as an orphan receptor. Owing to GCNF's unique features and its distant relation to any other known nuclear receptor it has been classified as the only member of the subgroup six and designated NR6A1 by the Receptor Nomenclature Committee (Duarte et al., 2002: Nucleic Acids Res 30: 364-368). To date, GCNF has been cloned from distinct vertebrate species, including zebrafish, Xenopus laevis, mouse, rat, and human. Cloning and characterization of the gene, domain organization and DNA binding properties of the protein, as well as the differential expression of mRNA splice variants or the protein during development and in the adult animal have been comprehensively reviewed by others (Greschik and Schüle, 1998: J Mol Med 76:800-810; Cooney et al., 1999: Am Zool 39:796-806). In this minireview I focus on the pleiotropic function of GCNF in embryogenesis and germ cell differentiation, and discuss novel concepts about its putative role in neurogenesis.
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