Copy number variants (CNVs) are common in normal individuals and have been identified in ~35% of the human genome.1 When present as hemizygous events in normal individuals, these imbalances are considered “benign” (i.e., no major phenotypic effect on human development); however, their role as susceptibility loci in common and complex genetic diseases and traits is now being actively explored. Data from control populations are being collected in databases of normal variation, including the Database of Genomic Variants (DGV)1 and the Database of Genomic Structural Variation (dbVar).3 These large datasets will contribute to a human gene dosage map through exclusion by defining those regions for which single copy loss or gain are tolerated and do not produce an overtly abnormal phenotype.
