Interestingly, not only functional deficiency of MeCP2 protein (i.e., due to mutations of MECP2) but also increased dosage of MeCP2 protein (i.e., due to duplication of MECP2) results in severe mental retardation in males [53] and cognitive impairment with learning difficulties and speech delay in females [54]. The gene discussed is MECP2; the disease is Cognitive impairment.