Several new microdeletion and microduplication syndromes have been discovered in a genotype-first approach. Many of these disorders are caused by nonallelic homologous recombination between blocks of segmental duplication. The authors describe 9 regions for which copy number alteration is proposed to cause an abnormal phenotype. Some of these disorders have been observed in affected individuals and individuals lacking a clearly abnormal phenotype. These deletions and duplications are thought to be contributory, but not always sufficient, to elicit an abnormal outcome. Additional studies are necessary to further evaluate the penetrance and delineate the clinical spectrum associated with many of these newly described disorders.