For many years neurodevelopmental disorders (NDDs) associated with mutations in genes located on the X-chromosome were thought to be causative only in males, while females were simply carriers. We and other have shown that especially de novo mutations in some X-chromosomal genes can cause NDDs in also females, e.g. in NAA10 (Saunier et al. 2016, Popp et al. 2015 ) or PHF6 (Zweier et al. 2013).