Use of N-Acetylcysteine in the Treatment of Repetitive and Self-Injurious Behaviors in Cornelia de Lange Syndrome

Inclusion Criteria: * Ages 13 to 35 years * A diagnosis of CdLS as determined by a physician during routine care meeting the major and minor criteria from CdLS guidelines * Threshold criteria for the presence of RB/SIB as reported on initial screening Children's Yale-Brown Obsessive Compulsive Scale Modified for Pervasive Developmental Disorders (CYBOCS-PDD) \> 6 OR Aberrant Behavior Checklist (ABC) stereotypy subscale \> 7) * Being able to attend 4 visits over the course of 18 weeks at the Johns Hopkins Hospital * No acute safety concerns or need for hospitalization due to psychotic, manic or depressive episode * Not currently pregnant or lactating/breastfeeding. Whether a participant is pregnant or not will be determined by the participant/caregiver report based on date last menses. If there is any suspicion of pregnancy, the PI will confer with the family to obtain testing through the primary care provider. Exclusion Criteria: * Allergy to NAC * Allergy to Quinine * Contraindication to NAC (organ transplant; untreated or symptomatic gastric condition) * Need for another medication with which NAC is contraindicated (antibiotics)