Array-based comparative genomic hybridization (aCGH) has become
a standard method for detecting copy-number variations (CNVs)
and other genetic defects associated with genetic diseases. Once
primarily the domain of postnatal testing for developmental
disorders, the technique is branching into prenatal (and
preconception) testing and cancer. aCGH involves labeling test
and reference genomes with differently colored dyes, mixing
them, then hybridizing the samples to a microarray of probes
covering a genomic area of interest. The scope may range from a
few hundred genes to the entire genome. aCGH differs from
standard CGH in its resolution. Because conventional CGH
hybridizes samples directly to chromosomes, resolution is
limited to five megabases or so. Arrays employ not whole
chromosomes but probes consisting of 50 to 75 bases. The densest
commercially available arrays today consist of between two and
three million probes. More common are 180,000-spot slides, which
provide whole-genome analysis at a resolution