NR0B1 (also known as DAX-1) is an orphan nuclear receptor involved in the development and maintenance of the steroid hormone pathway. It also plays a role in the development of the embryo and maintenance of pluripotent embryonic stem cells []. Mutations of the DAX-1 gene cause X-linked adrenal hypoplasia congenita (XL-AHC), a developmental disorder of the adrenal gland that results in profound hormonal deficiencies and is lethal if untreated [].NR0B2 lacks a conventional DNA binding domain (DBD) and represses the transcriptional activity of various nuclear receptors [].Steroid or nuclear hormone receptors (NRs) constitute an important superfamily of transcription regulators that are involved in widely diverse physiological functions, including control of embryonic development, cell differentiation and homeostasis. Members of the superfamily include the steroid hormone receptors and receptors for thyroid hormone, retinoids, 1,25-dihydroxy-vitamin D3 and a variety of other ligands []. The proteins function as dimeric molecules in nuclei to regulate the transcription of target genes in a ligand-responsive manner [, ]. In addition to C-terminal ligand-binding domains, these nuclear receptors contain a highly-conserved, N-terminal zinc-finger that mediates specific binding to target DNA sequences, termed ligand-responsive elements. In the absence of ligand, steroid hormone receptors are thought to be weakly associated with nuclear components; hormone binding greatly increases receptor affinity.NRs are extremely important in medical research, a large number of them being implicated in diseases such as cancer, diabetes, hormone resistance syndromes, etc. While several NRs act as ligand-inducible transcription factors, many do not yet have a defined ligand and are accordingly termed 'orphan' receptors. During the last decade, more than 300 NRs have been described, many of which are orphans, which cannot easily be named due to current nomenclature confusions in the literature. However, a new system has recently been introduced in an attempt to rationalise the increasingly complex set of names used to describe superfamily members.
