Mutations of the human androgen receptor gene may disturb sexual development in males, and are inherited as an X-linked recessive trait. The vast majority of the mutations are familial. We have identified a large kindred with complete androgen insensitivity syndrome (CAIS) without detectable androgen-binding in genital skin fibroblasts. A single nucleotide substitution (C-to-T transition) was identified, resulting in an Arg855 to Cys in the androgen binding domain. To date, four independent CAIS families have been reported with this specific mutation that coincides with the propensity of cytosines at CpG dinucleotides to methylate. An allele-specific oligo-nucleotide assay was developed that allowed for the rapid and specific identification of this mutation hot-spot in individuals with androgen receptor insensitivity syndromes.