MECP2 and X-linked intellectual disability: The ARX (Aristaless-related homeobox) encodes a transcription factor important for neuronal development [100], whose loss-of-function variants/mutations contribute to X-linked intellectual disability and epilepsy [101,102], while increasing the gene copy number of the other X-linked gene MECP2 (methyl-CpG binding protein 2) leads to MECP2 duplication syndrome (MDS) [103].