Phase 2 is a double-blinded, placebo-controlled cross-over study designed to test the efficacy of IGF-1 for treatment of Rett syndrome. The Nelson Lab uses both EEG and a cognitive developmental assessment to evaluate changes in cortical function across treatment. For the EEG, we do this by evaluating two well characterized systems in the brain (the auditory system and the visual system). We use traditional ERP paradigms as well as steady-state presentation of auditory and visual stimuli and power spectra analysis to assess cortical function of V1 and auditory cortex. In addition, the Nelson Lab has adapted the Mullen Scales of Early Learning for use with children with Rett syndrome.