Another severe X‐linked neurodevelopmental disorder is Rett syndrome (RTT), that is caused by mutations in the transcriptional regulator MECP2, an X chromosome gene. Similar to FXS, symptoms typically include language delays, motor coordination problems and repetitive movements. 59 Originally Rett was thought to be fatal in males, and only manifested in females. However, like other X chromosome genes, MECP2 is subject to X‐inactivation, and most affected individuals are female heterozygotes who display cellular mosaicism for normal and mutant MECP2. Males rarely survive, but those who do, and are hemizygous for mutant MECP2 are more severely affected than heterozygous females. 60
