Genetic sex in mammals is determined by the sex chromosomal composition of the zygote. The X and Y chromosomes are responsible for numerous factors that must work in close concert for the proper development of a healthy sexual phenotype. The role of androgens in case of XY chromosomal constitution is crucial for normal male sex differentiation. The intracellular androgenic action is mediated by the androgen receptor (AR), and its impaired function leads to a myriad of syndromes with severe clinical consequences, most notably Androgen insensitivity syndrome (AIS). AIS is a condition that results in the partial or complete inability of the cell to respond to androgens, so the animal does not become masculinized. This results in XY (genetically male) horses with female phenotype (female external genitalia and internal testis) with male-like behavior. These individuals are unable to reproduce.