Rett syndrome, an inherited neurological disorder, is one of the most common causes of mental retardation in females. Babies with Rett syndrome develop normally until 6–18 months, at which point they show a reduction in brain size and become prone to seizures and autism. The syndrome is caused by a mutation in the MECP2 gene, which maps to the X chromosome. Females with one intact copy of the gene survive until adulthood, despite the neurological symptoms. Male embryos that carry the mutation die during development. The MECP2 gene encodes the methyl-CpG-binding protein 2, which binds to methylated sites in the genome and represses transcription of adjacent genes.