MECP2 genetic testing should be performed in all girls with ASD and MR, remembering that clinicians should not expect these patients to bear any resemblance to Rett syndrome (ie, girls with autism carrying MECP2 mutations will have normal head size, a history of regression may be present or absent, and they will usually show no stereotypies or seizures). This evidence concerns the gene MECP2 and atypical Rett syndrome.