During the early stages of embryo growth, the SRY gene located on the Y chromosome triggers male sexual development. Typically, embryos with two X chromosomes develop into females. We report a child with two X chromosomes who had male appearing genitalia (female-to-male sex reversal) due to a deletion of a small X chromosome segment controlling testis determination. Our study shows how gene dosage and disrupted gene regulation can switch gonadal development to activate male differentiation in the absence of the Y chromosome. This case provides new insight into the genetic causes of disorders of sexual development in humans. (By Dr. Svetlana A. Yatsenko, http://jmg.bmj.com/content/early/2017/05/07/jmedgenet-2016-104128 )