DAX-1 in sex determination and gonadal development: revisiting the anti-testis hypothesis.

DAX-1 (NR0B1) is a pivotal regulator of mammalian reproductive and endocrine development. Discovered as a gene whose dosage imbalance can reverse sex in XY individuals, DAX-1 is now recognized as a key component of the network controlling gonadal formation and function. Despite decades of research, the molecular mechanisms of its function in gonad development and function remain only partially understood. We combine genetic, developmental, molecular and biochemical perspectives to present how DAX-1 regulates gene expression from early sex determination and gonadal differentiation to the maintenance of steroidogenic homeostasis in adrenal and gonadal tissues. Recent advances have reshaped our view of DAX-1 beyond its classical role as a transcriptional repressor. Evidence shows that it functions within dynamic transcriptional networks, interacting with regulators such as SF-1 (NR5A1), SOX9 and other nuclear receptors to fine-tune gene expression in a context-dependent manner. Genomic, structural, and proteomic studies reveal that DAX-1 participates in complexes integrating developmental and hormonal cues, coordinating differentiation and steroidogenesis. Moreover, its emerging roles in RNA-associated gene regulation broaden its functions beyond canonical nuclear receptor signaling. We revisit DAX-1 contribution to sex determination and differentiation from an evolutionary and mechanistic viewpoint, emphasizing its interactions with the SRY-SOX9 axis and its dosage-sensitive influence on gonadal fate. The developmental and clinical consequences of DAX-1 mutations and copy number variations manifest as a spectrum of human phenotypes, from adrenal hypoplasia congenita to differences/disorders of sex development (DSD), highlighting its essential role in maintaining endocrine balance.