Mutations in the DAX-1 [dosage-sensitive sex reversal-adrenal hypoplasia congenita (AHC) critical region on the X chromosome; NR0B1] gene cause X-linked AHC associated with hypogonadotropic hypogonadism. DAX-1 encodes an unusual orphan member of the nuclear hormone receptor superfamily, acting as a transcriptional repressor of genes involved in the steroidogenic pathway. All DAX-1 mutations found in AHC patients alter the protein C terminus, which shares similarity to the ligand binding domain of nuclear hormone receptors and bears transcriptional repressor activity. This property is invariably impaired in DAX-1 AHC mutants. Here we show that the localization of DAX-1 AHC mutant proteins is drastically shifted toward the cytoplasm, even if their nuclear localization signal, which resides in the N terminal of the protein, is intact. Cytoplasmic localization of DAX-1 AHC mutants correlates with an impairment in their transcriptional repression activity. These results reveal a critical role of an intact C terminus in determining DAX-1 subcellular localization and constitute an important example of a defect in human organogenesis caused by impaired nuclear localization of a transcription factor. A drenal hypoplasia congenita (AHC) is an inherited disorder of the adrenal cortex commonly manifested as an earlyonset adrenal insufficiency syndrome. Two different forms of this disease have been recognized: a X-linked form and a rare autosomal recessive or sporadic form (1). AHC is a lethal disease if untreated, mainly because of mineralocorticoid deficit. Although it can present a wide clinical spectrum (1-3), a constant feature of the X-linked syndrome is the association with hypogonadotropic hypogonadism (HHG). It has been shown that HHG is caused by combined hypothalamic failure in gonadotropin-releasing hormone release and pituitary defect in gonadotropin production (2). Mutations in the DAX-1 (dosagesensitive sex reversal-AHC critical region on the X chromosome gene 1; NR0B1) gene, situated in Xp21, are responsible for the X-linked form of AHC͞HHG (4, 5).DAX-1 encodes an unusual orphan member of the nuclear hormone receptor superfamily (4-7) whose expression is restricted to the adrenal cortex, testis, ovary, pituitary gonadotropes, and hypothalamic ventromedial nucleus (8,9). This pattern of expression closely matches the characteristics of the disease and suggests that DAX-1 also plays a role in reproductive function. Strikingly, inactivation of the mouse Ahch homologue does not produce adrenal hypoplasia, but male sterility caused by progressive testicular seminiferous tubules degeneration (10). Other links with reproductive function and phenotypical sex determination are provided by the findings that the DAX-1 gene lies inside the critical region on the X chromosome whose duplication causes dosage-sensitive sex reversal in XY individuals (11,12), and that overexpression of DAX-1 in particular mouse strains causes phenotypical sex reversal (13). Furthermore, mouse Dax-1 has been shown to antagonize the synergisti...