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Little girls are made of sugar and spice and, according to a study published in the journal Cell, a fierce determination to maintain their girlishness. Because it seems that a single gene¹ keeps their ovaries from turning into testes. 

Scientists have long thought that the ovary is the default setting for developing sex organs. That’s because the male Y chromosome² comes equipped with a so-called sex determination gene called SRY. If SRY is missing, the gonad develops into an ovary, even if the embryo³ is male. Add SRY to a female embryo, it’ll make testes. But the new work implies that sex determination is not that simple. 

Okay, buckle⁴ up. See, the SRY gene makes a male by turning on another gene called Sox9. But it turns out that females have a trump⁵ card called FOX L2. FOX L2 switches off Sox9, which lets ovaries be ovaries. The surprise is that girls have to keep the FOX L2 gene active forever. In studies of mice, if FOX L2 gets shut off, even in an adult, the ovaries will turn into testes. Which for a girl means ball game over.

—Karen Hopkin