Neurophysiologic Biomarkers in Rett Syndrome

Inclusion Criteria: 1. Rett Group: Females ages 3-18 (inclusive) with a clinical diagnosis of RTT with a likely pathogenic or known pathogenic variant in MECP2. 2. Likely Rett Group: Females from 1 year to \< 5 years of age with MECP2 variant if regression has not yet occurred or child is within 6 months of last skill loss. 3. Typically developing (TD) Group: Females age matched to RTT population (1-18) with no developmental or cognitive concerns as assessed using the Child/Adult Behavioral Checklist, Survey of Well-Being of Young Children (\<5yo), or the Wide Range Achievement Test-4 (\>5 yo). Exclusion Criteria: Rett and Likely Rett Groups: 1. Presence of a duplication in MECP2 or any other identified pathogenic variant in another gene with a neurodevelopmental phenotype. 2. Active medical conditions not typically found in RTT. Typically Developing Group: 1. Score below norms on the performance tests 2. Have a known neurological disorder (excluding migraine) 3. Being on neuroactive medications.