XX = girl and XY = boy
That's what we learnt in school, right? However, there is more to gender phenotype than meets the eye.
We have recently uncovered the molecular mechanism underlying sexual maintenance in mammals revealing an unexpected and fascinating plasticity through a Yin and Yang relationship between two genes, whose expression is mutually exclusive. These two genes are called FOXL2 and SOX9, both transcriptional regulators.
While SOX9 protein expression is abundant in males at all stages of development, it has to be continuously suppressed in adult females to stabilize the female phenotype and to prevent trans-differentiation of the ovaries into testes (see Uhlenhaut et al, Cell 2009).