The USP9Y gene is found on the azoospermia factor (AZF) region on the Y chromosome. Men who have impaired or no sperm production often have a deletion in the AZF region, especially in the USP9Y gene, and it was thought that USP9Y was necessary for sperm production. However, a man and his father with a USP9Y deletion who could produce sperm were recently reported. The corresponding gene is present but inactive in chimpanzees and bonobos.
