Müllerian inhibiting substance (MIS), also known as anti-Müllerian hormone (AMH), is a protein produced primarily by the testes during male fetal development. It plays a crucial role in sexual differentiation by inhibiting the formation of Müllerian ducts, which would otherwise develop into female reproductive structures such as the uterus and fallopian tubes. In females, MIS is produced in smaller amounts by the ovaries and is involved in regulating ovarian function. Measuring MIS levels can provide insights into ovarian reserve and reproductive health, making it a valuable marker in fertility assessments and treatments.