Cornelia de Lange syndrome is a pleiotropic developmental syndrome characterized by growth and cognitive impairment, facial dysmorphic features, limb anomalies, and other malformations. Mutations in core cohesin genesSMC1AandSMC3, and the cohesin regulatory gene,NIPBL, have been identified in Cornelia de Lange syndrome probands. Patients withNIPBLmutations have more severe phenotypes when compared to those with mutations inSMC1AorSMC3. To date, 26 distinctSMC1Amutations have been identified in patients with Cornelia de Lange syndrome. Here, we describe a 3–year–old girl with psychomotor and cognitive impairment, mild facial dysmorphic features but no limb anomaly, heterozygous for a c.1487G>A mutation inSMC1Awhich predicts p.Arg496His. We show that this mutation leads to an impairment of the cellular response to genotoxic treatments. © 2011 Wiley Periodicals, Inc.