AIS is caused by genetic defects on the X chromosome. These defects make the body unable to respond to the hormones that produce a male appearance.

The syndrome is divided into two main categories:

Complete AIS

Partial AIS

In complete AIS, the penis and other male body parts fail to develop. At birth, the child looks like a girl. The complete form of the syndrome occurs in as many as 1 in 20,000 live births.

In partial AIS, people have different numbers of male traits.

Partial AIS can include other disorders, such as:

Failure of one or both testes to descend into the scrotum after birth

Hypospadias, a condition in which the opening of the urethra is on the underside of the penis, instead of at the tip

Reifenstein syndrome (also known as Gilbert-Dreyfus syndrome or Lubs syndrome)

Infertile male syndrome is also considered to be part of partial AIS.