Sex-determining region Y protein
'Sex-determining region Y protein' (SRY), or testis-determining factor (TDF), is a DNA-binding protein (also known as gene-regulatory protein/transcription factor) encoded by the SRY gene that is responsible for the initiation of male sex determination in therian mammals (placental mammals and marsupials). SRY is an intronless sex-determining gene on the Y chromosome. Mutations in this gene lead to a range of disorders of sex development with varying effects on an individual's phenotype and genotype. SRY is a member of the SOX (SRY-like box) gene family of DNA-binding proteins. When complexed with the (SF-1) protein, SRY acts as a transcription factor that causes upregulation of other transcription factors, most importantly SOX9. Its expression causes the development of primary sex cords, which later develop into seminiferous tubules. These cords form in the central part of the yet-undifferentiated gonad, turning it into a testis. The now-induced Leydig cells of the testis then start secreting testosterone, while the Sertoli cells produce anti-Müllerian hormone. SRY gene effects normally take place 6–8 weeks after fetus formation which inhibits the female anatomical structural growth in males. It also works towards developing the secondary sexual characteristics of males. SRY may have arisen from a gene duplication of the X chromosome bound gene SOX3, a member of the SOX family. This duplication occurred after the split between monotremes and therians. Monotremes lack SRY and some of their sex chromosomes share homology with bird sex chromosomes. SRY is a quickly evolving gene, and its regulation has been difficult to study because sex determination is not a highly conserved phenomenon within the animal kingdom. Even within marsupials and placentals, which use SRY in their sex determination process, the action of SRY differs between species. The gene sequence also changes; while the core of the gene, the high-mobility group (HMG) box, is conserved between species, other regions of the gene are not.
