GWAS technologies can also detect more of the copy number variants (CNVs) in the genome than was possible with older cytogenetic methods, by analysis of the relative intensities of the fluorescent labels used in the assays. CNVs are deletions and duplications of DNA segments, of diverse sizes and population frequencies. For example, large deletions on chromosome 22q11 cause the velocardiofacial/DiGeorge syndrome, and 20% of such cases also develop schizophrenia.(36) CNVs tend to arise in regions with repetitive DNA sequences. Some CNVs are common and are transmitted from generation to generation, while others recurrently arise de novo. Like rare SNPs, rare CNVs are more likely to be harmful. (Other structural variants such as inversions and translocations remain difficult to detect.) Large genomewide CNV scans show that CNVs are more common than was previously recognized. (37) Structural variation has not been as comprehensively studied as SNPs, because CNV detection is less accurate, biological confirmation is still costly, and smaller CNVs (less than 100,000 base pairs) are less reliably detected. But technologies are rapidly improving. Significant CNV findings are now being reported for psychiatric disorders as discussed below.
