Chromosomal rearrangements have been reported in a small number of individuals with OCD [2-7]. Two microdeletion disorders, 22q11.2 deletion syndrome and Prader-Willi syndrome (PWS), due to deletion of chromosome 15q11-13, are frequently associated with obsessive-compulsive symptoms, suggesting that gene dosage effects at these two loci could contribute to the development of the obsessive-compulsive phenotype. 22q11.2 deletion syndrome, also known as DiGeorge or velocardiofacial syndrome, is a highly variable disorder caused by a microdeletion of chromosome 22q11.2, often occurring de novo (80%), with an estimated prevalence of 1/2000-6000 live births [8,9]. The majority of 22q11.2 deletions (87%) are ~3 Mb in size, whereas a small proportion (8%) involves smaller nested ~1.5 Mb deletions [10] (Figure 1). 22q11 deletion syndrome is characterized by cardiac malformations, immunodeficiency, dysmorphic features, and palate anomalies, associated with cognitive impairment. Comorbid psychophathologies are frequent, and include schizophrenia, attention deficit-hyperactivity disorder, autism spectrum disorders and OCD or obsessive-compulsive symptoms [11]. In particular, 33% (14/43) of 22q11.2 deletion subjects meet criteria for OCD and an even higher proportion report obsessive-compulsive symptoms (83%) [12], suggesting that the 22q11.2 locus harbors
