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XX male syndrome, also known as De la Chapelle syndrome, is a rare congenital intersex condition where an individual with a female genotype has phenotypically male characteristics that can vary between cases. In 90 percent of these individuals, the syndrome is caused by unequal crossing over between X and Y chromosomes during meiosis in the father. This results in the X chromosome containing the SRY gene, as opposed to the Y chromosome where it is normally found. When the X chromosome containing the SRY gene combines with a normal X from the mother during fertilization, the result is an XX genotype with male sex characteristics. Less common are SRY-negative XX males which can be caused by a mutation in an autosomal or X chromosomal gene. The masculinization of XX males is variable.

This syndrome is diagnosed through various detection methods and occurs in approximately 1:20 000 newborn males, making it much less common than Klinefelter syndrome. Treatment is medically unnecessary, although some individuals choose to undergo treatments to make them appear more male or female.

XX male syndrome is not to be confused with sex determination in some non-human mammal species that lack a Y chromosome at all, such as the Transcaucasian mole vole (Ellobius lutescens).