（image source：Asato Kuroiwa）
In mammals, the sex of an individual usually depends on the X and Y chromosomes, usually XX for females and XY for males.
Among them, the Sry gene on the Y chromosome promotes the development of the testis and is therefore important for the sexual differentiation of males.
However, there are a few rodents that lack the Y chromosome, such as the Amami rat (Tokudaia osimensis), which has only one X chromosome in both sexes but still develops testes.
A recent study published in the Proceedings of the National Academy of Sciences (PNAS) reveals the mechanism of testicular development in the amelogenous mouse.
The researchers analyzed the genome sequences of three pairs of female and male amastigotes and identified a DNA repeat sequence that is present only in males.
This repeat sequence is located upstream of the Sox9 gene sequence on chromosome 3 and promotes the expression of the Sox9 gene in the absence of the Sry gene.
Further studies revealed that the effect of this repeat sequence is similar to that of the Sox9 enhancer (capable of enhancing gene expression), Enh14, and may act synergistically with it.
After the sequence was introduced in female descent mice by gene editing, female descent mice embryos showed expression of the gene that induces testis formation, suggesting that the sex of Amami mice lacking the Y chromosome can be determined by chromosome 3.