The purpose of this study is to pilot the use of Insulin-Like Growth Factor-1 (IGF-1) treatment in 22q13 Deletion Syndrome (Phelan-McDermid Syndrome) caused by SHANK3 gene deficiency in order to evaluate safety, tolerability, and efficacy. IGF-1 is an injection under the skin that contains human IGF-1. IGF-1 is approved by the FDA under the brand name Increlex for the treatment of children with short stature due to primary IGF-1 deficiency. It is being used off-label in the current study and is not FDA approved, nor has it yet been studied in humans for the treatment of SHANK3 deficiency.

Overall, there will be 1-3 screening visits, a baseline visit where study drug will first be administered, and then 10 follow-up visits. Follow-up visits will occur at week 2, week 4, week 8, and week 12 in each treatment phase (IGF-1 or placebo), and then again 4 weeks after study completion, Parents/guardians will be asked to administer the IGF-1/ placebo by injection at home and will also be responsible for monitoring glucose levels in the child. Parents/guardians will be trained in these methods, and will have scheduled phone calls and appointments where the dose and tolerability will be discussed. Assessments include the following: * Physical and neurological examination * Medical and psychiatric history * X-ray of long bone (e.g., hand) to ensure your child's growth plates are not closed * Electrocardiography * Echocardiography * Pregnancy test if applicable * Lab safety measures (through blood draw) * Autism Diagnostic Interview (ADI) * Autism Diagnostic Observation Schedule (ADOS) * The Mullen Scales of Early Learning or the Leiter International Performance Scale-Revised * Vineland Adaptive Behavior Scale (VABS) * Clinical Global Impressions (CGI) Rating Scales * The Repetitive Behaviors Scale (RBS) * Aberrant Behavior Checklist (ABC) * The Caregiver Strain Questionnaire (CSI) * Language Environment Analysis (LENA) * The Macarthur-Bates Communication Inventory (MCDI) * Unified Parkinson's Disease Rating Scale (UPDRS) * Quick Neurological Screening Test 2nd Edition (QNST-2) * Gait Analysis with motion capture video systems and interactive 3-dimensional modeling systems