 For example, MECP2 mutations are associated with some cases of moderate to severe X-linked mental retardation.Mutations in the gene have also been found in males with severe brain dysfunction (neonatal encephalopathy) who live only into early childhood.In addition, several people with features of both Rett syndrome and Angelman syndrome (a condition characterized by mental retardation, problems with movement, and inappropriate laughter and excitability) have mutations in the MECP2 gene.