XX male syndrome

XX male syndrome, also known as de la Chapelle syndrome or 46,XX testicular disorder of sex development (or 46,XX DSD) is a rare intersex condition in which an individual with a 46,XX karyotype develops a male phenotype.

In 90 percent of these individuals, the syndrome is caused by the Y chromosome's SRY gene, which triggers male reproductive development, being atypically included in the crossing over of genetic information that takes place between the pseudoautosomal regions of the X and Y chromosomes during meiosis in the father. When the X with the SRY gene combines with a normal X from the mother during fertilization, the result is an XX genetic male. Less common are SRY-negative individuals, who appear to be XX genetic females, which is caused by a mutation in an autosomal or X chromosomal gene. Masculinization in those with the condition is variable, and those with the condition are sterile.

This syndrome is diagnosed and occurs in approximately 1:20,000 newborn boys, making it much less common than Klinefelter syndrome. Medical treatment of the condition varies, with medical treatment usually not necessary. The clinical name "de la Chapelle syndrome", was named after the Finnish scientist Albert de la Chapelle, who first described the condition.