a genetic factor, called testis-determining factor (TDF), which was identified more than 20 years ago as SRY , a gene carried by the Y chromosome. First, SRY triggers a cascade of molecular events, the SOX9 pathway, inducing the undifferentiated urogenital ridge, a gonadal primordium characterizing all embryos, to differentiate into testis. Successively, some hormonal products from specific testis cells, i.e. anti-Müllerian hormone (AMH) by Sertoli cells and testosterone by Leydig cells, head

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... le phenotype development of the embryo. In the absence of SRY , a specific female molecular cascade, the ␤ -catenin pathway, directs development of ovaries and female phenotype differentiation. First Steps in Sex Determination In both male and female murine embryos, the first sign related to sex development occurs at about 10.0 days post coitum (dpc) when a thickening of the epithelium, which gonadal somatic cells will derive from, appears along the coelomic surface of the mesonephros [Ross and Capel, 2005] . It is exactly when the XX and XY gonadal primordia show gonadal-specific gene expression, which is not sexually dimorphic until the activation of the malespecific pathway at 10.5 dpc. Knockout (KO) experiments in mice have determined that some of these genes are responsible for the early proliferation of gonads in both sex-Abstract Gonadal cellular organization is very similar in all vertebrates, though different processes can trigger bipotential gonads to develop into either testes or ovaries. While mammals and birds, apart from some exceptions, show genetic sex determination (GSD), other animals, like turtles and crocodiles, express temperature-dependent sex determination. In some groups of animals, GSD can also be overridden by hormone or temperature influences, indicating how fragile this system can be. This review aims to explain the fundamental molecular mechanisms involved in mammalian GSD, mainly referring to mouse as a major model. Conceivably, other mammals might show a molecular mechanism different from the commonly investigated murine species.