For the 14 recurrent regions, the number of deletions and duplications were often unequal, which can be explained by ascertainment (recurrent duplications may result in milder phenotypes and therefore not be ascertained in our cohort of affected individuals) and mechanism (deletions generated by NAHR occur more frequently than duplications)49. Not surprisingly, the most common deletion in this cohort, with 93 cases (1 in 169 abnormal cases), was the 22q11.2 deletion (MIM 188400),40 while the reciprocal duplication (MIM 608363) with a milder phenotype 41 was detected in only 32 cases. The most common recurrent duplication in our dataset was in 16p13.11, seen in 45 cases, while the reciprocal deletion associated with neurodevelopmental defects was detected in only 22 cases. For both deletions and duplications, the second most commonly affected region was the recurrent 16p11.2 CNV (MIM 611913). Both deletions and duplications of this region have been reported in individuals with an abnormal neurological phenotype.30 The frequency of the 16p11.2 deletion in this abnormal cohort is approximately 1 in 235. Therefore, this CNV was detected nearly as often as the 22q11.2 deletions, indicating that this CNV is also a frequent cause of intellectual and developmental disabilities.
