Complete androgen insensitivity syndrome (CAIS) is a genetic condition in which a child is genetically male, but develops female sex characteristics. CAIS is a disorder of sex development (DSD).

When an egg cell is fertilized by a sperm cell, each parent contributes a sex chromosome. These chromosomes determine whether a baby is a girl or a boy. Females have two X chromosomes (XX). Males have one X chromosome and one Y chromosome (XY).

Genes, located on chromosomes, contain genetic material that provide a “road map” for how the child will develop.

CAIS occurs when there is a problem with one of the genes on the X chromosome. This specific gene is called an androgen receptor gene. It governs how a developing fetus responds to androgens – hormones that bring about male characteristics.

A child with CAIS has a genetic makeup of XY. Because the Y chromosome is present, the child is born with testes, just like a boy, although the testes are undescended (located inside the abdomen). But because of the defective gene on the X chromosome, other male characteristics can’t develop, so the child resembles a girl.

Most children with CAIS are raised as females. They usually have their testes surgically removed because undescended testes can become cancerous later on. They may need treatment to develop the vagina. They develop breasts but do not have menstrual periods and may have very little pubic and/or underarm hair. They may also need to take estrogen, a female hormone, for the rest of their lives.

CAIS should not be confused with partial androgen insensitivity syndrome (PAIS). Children with PAIS can respond to androgens to some degree, but not enough to develop fully into a male or female. As a result, they have both male and female physical characteristics.