Sexual differentiation in humans development can only occur when the fetal testis secretes key hormones at a critical period in early gestation. The testes begin to secrete three hormones that influence the male internal and external genitalia: they secrete anti-müllerian hormone (AMH), testosterone, and dihydrotestosterone (DHT). Anti-müllerian hormone causes the paramesonephric ducts to regress. Testosterone converts the mesonephric ducts into male accessory structures, including the epididymis, vas deferens, and seminal vesicle. Testosterone will also control the descending of the testes from the abdomen into the scrotum. Many other genes found on other autosomes, including"WT-1", "SOX9", "SF-1" also play a role in gonadal development. Females:
Transgender hormone therapy (male-to-female) suggesting a protective role of the male sex-determination chromosome. On the other hand, men with Klinefelter's syndrome (two X chromosomes and one Y chromosome), which causes hypoandrogenism, hyperestrogenism, and a very high incidence of gynecomastia (80%), have a dramatically (20- to 58-fold) increased risk of breast cancer compared to men with one X chromosome, closer to the rate of homogametic females. The incidences of breast cancer in karyotypical men (46,XY karyotype), men with Klinefelter's syndrome (47,XXY karyotype), and karyotypical women (46,XX karyotype) are approximately 0.1%, 3%, and 12.5%, respectively. Individuals with the 46,XY karyotype affected by complete androgen insensitivity syndrome