The hypothesis of “female X-linked two-hit model” has been used to support studies on multiple disorders, including X-linked intellectual disability (Plenge et al., 2002), MECP2 duplication (Fieremans et al., 2014), and the Wiskott-Aldrich syndrome (Daza-Cajigal et al., 2013). The gene discussed is MECP2; the disease is Wiskott-Aldrich syndrome.