In a genetically male (XY) foetus the active intervention of male hormones (androgens) is needed to produce a fully male system. In androgen insensitivity syndrome (AIS) the child is conceived with male (XY) sex chromosomes. Embryonic testes develop inside the body and start to produce androgens but these cannot be used by the body to complete male genital development. The development of the external genitals continues along female lines. However, another hormone produced by the foetal testes suppresses the development of female internal organs. Thus a person with AIS has external genitals that in Complete AIS (CAIS) are completely female or in Partial AIS (PAIS) are partially female. Internally, however, there are testes instead of a uterus and ovaries.
AIS is an example of a biological intersex condition, in which the reproductive organs/genitalia differ from the person's genetic sex. This is not the same as being a transsexual (gender dysphoria).
In about two thirds of all cases, AIS is inherited from the mother. In the other third there is a spontaneous mutation in the egg.