There are 2 types of sex chromosomes in humans: X and Y. Chromosomal sex is male when a Y chromosome is present (e.g., 46,XY or 47,XXY) and female when the Y chromosome is absent (e.g., 46,XX or 45,X0). Male phenotypes develop when a specific gene, called the SRY gene (usually found on the Y chromosome), is present, stimulating differentiation of the gonads into testes. The testes then produce testosterone (triggering development of the penis and scrotum externally and the ejaculatory system internally) and antimüllerian hormone (AMH), which causes regression of the müllerian ducts. Without the SRY gene, ovaries develop; without testosterone, external female genitalia develop; and without AMH, the müllerian ducts persist and differentiate into the fallopian tubes, uterus, and upper vagina.