Autism Spectrum Disorders (ASDs) are complex neurodevelopmental conditions affecting 1/80 children. With recent advances in genetic association and functional gemonics, the neurobiology of these disorders is emerging. However, current treatments aim at managing the symptoms rather than treating underlying disease biology. Some forms of ASD are caused by mutations in a single gene, and are said to be monogenic. One example is Rett Syndrome (RS), which in most cases is caused by mutations in the mecp2 gene (also mutated in other ASDs). Rett syndrome is an ASD manifesting around 6-18 months of age after an apparently normal development. Regression of speech and motor abilities, impaired upper limb function and cardio-respiratory abnormalities dominate the clinical picture. Treatment of Rett syndrome represents a largely unmet clinical need. IGF1 is a promising treatment for Rett syndrome, and clinical trials are in progress to test its efficacy (NCT01777542). Furthermore, two additional clinical trials on ASD patients have started (NCT01525901, NCT01970345). The safety of IGF1 treatment has been demonstrated in Rett syndrome patients with mixed treatment responses (Pini et al., 2012; Khwaja et al., 2014). In prelimary studies, several subjects showed significant improvement, whilst others showed an intermediate level of benefits, and some patients showed no beneifts (Pini et al., 2012; Khwaja et al., 2014). These observations suggest that treatment response may differ between patients for different clinical parameters. The present study aims to correlate clinical and molecular parameters with treatmnet outcomes to uncover possible biomarkers and identify patients that would benefit from IGF1 treatment.