This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Rett syndrome (RS) is a neurodevelopmental disorder that develops almost exclusively in females following apparently normal psychomotor development for the first six months of life. The characteristic features include loss of speech and purposeful hand use, occurrence of stereotypic hand movements, gait dyspraxia, and deceleration of head growth [1]. These individuals frequently develop severe motor problems including an abnormal gait or the loss of ability to ambulate. They may develop seizures, abnormal breathing consisting of periods of apnea and hyperventilation occurring only during wakefulness, symptoms suggesting autonomic nervous system dysfunction, and growth failure. Increases in occurrence of prolonged QTc (>0.45 secs) and abnormal heart rate variability consistent with clinical signs (e.g. cold, blue extremities) indicating autonomic dysfunction have been previously reported in Rett syndrome [2.3]. Recently, the gene for RS was discovered. Amir et al. [4] reported the presence of several mutations in MECP2 in individuals with RS. MECP2 encodes methyl-CpG-binding protein 2 (MeCP2). MeCP2 is a member of a family of proteins known to bind specifically to methylated CpCs and to be capable of repressing transcription. Although MeCP2 is expressed in all tissues, it is more abundant inthe brain than any other tissue, and the brain may be more sensitive to abnormal MeCP2 than other tissues. The binding site of MeCP2 requires only a single methylated CpG dinucleotide to bind. It has been proposed that MeCP2 acts as a global transcriptional repressor that prevents unscheduled transcription throughout the genome and has been implicated as a key player in assembling transcriptional silencing complexes. Approximately 80% of females meeting the clinical criteria for Rett syndrome willhave a mutation in MECP2. Hence, we expect to enroll a higher number of participants from among those who have such mutations. Over the past twenty years, investigators in this network have acuqired significant experience concerning Rett Syndrome. We have an established consortium of clinical investigators at the University of Alabama at Birmingham and the Baylor College of Medicine in Houston, TX. Members of this consortium were among the first to provide extensive characterization of the clinical aspects of Rett Syndrome including growth, nutrition, neurophysiology, epidemiology including survival, motor performance, behavior, and the neuropathology of this disorder. Huda Zoghbi, a member of the Baylor team, directed the effort identifying mutations in the Xq28 gene MECP2, encoding methyl-CpG- binding protein 2 as the molecular basis for Rett syndrome. We now know that the phenotypic consequences of MECP2 mutations range in females from normal or mild learning disability to classic Rett syndrome, dependin on the pattern of X-chromosome inactivation. In males, MECP2 mutations also produce variable clinical consequences, ranging from fatal encephalopathy in infancy to X-linked mental retardation. The consortium''s ongoing phenotype- genotpe study has characterized the clinical characteristics of several hundred females from age one to 55 years with Rett syndrome and assessed the presence or absence of MECP2 mutations. More than 85% of participants with classic Rett syndrome have such mutations.