Comparative genomic hybridization has revealed an additional level of genomic complexity through the identification of relatively common deletions and insertions throughout the genome in all individuals, some of which may be associated with disease. These copy number changes may be below the resolution of a standard G-banded karyotype that typically can distinguish only genomic alterations greater than 5 to 10 megabases (Figure, A). Depending on the array platform, deletions and insertions can now be resolved at 6000 or fewer base pairs (Figure, B). Current clinical applications for array comparative genomic hybridization include the evaluation of several hundred known regions of human variation of significance in disease (eg, Wolf-Hirschhorn syndrome [chromosome 4p−], Angelman syndrome, and DiGeorge Syndrome). Array comparative genomic hybridization studies may eventually replace the karyotype in the initial workup of individuals with dysmorphic features, malformations, or developmental delay. However, structural alterations of the genome, including balanced translocations, inversions, and point mutations, are not typically detected by array comparative genomic hybridization.