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Summary
How the physical differences between the sexes develop is an intriguing process. In mammals, the first important stage in sex determination occurs during fetal life, when the male-determining Y chromosome expresses a gene called sex determining region Y (SRY), which acts on the bipotential gonad to initiate testis (instead of ovary) formation (1). Landmark experiments by Jost in the 1950s showed that the development of male-specific structures is established by factors produced from the emerging testis, namely androgens and anti-Müllerian hormone (AMH) (2). This suggested that the formation of male characteristics is an active process, whereas female-specific differentiation is passive. On page 717 of this issue, Zhao et al. (3) show that female development is also an active process.
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