Editing Comparative genomic hybridization (section)

==Limitations of CGH and array CGH==
A main disadvantage of conventional CGH is its inability to detect structural chromosomal aberrations without [[copy number variant|copy number changes]], such as [[mosaic (genetics)|mosaicism]], balanced [[chromosomal translocations]], and [[chromosomal inversion|inversions]]. CGH can also only detect gains and losses relative to the ploidy level.<ref>Weiss MM, Hermsen MAJA, Meijer GA, van Grieken NCT, Baak JPA, Kuipers EJ, van Deist PJ (1999) Comparative genomic hybridization. J Clin Pathol: Mol Pathol 52:243–251.</ref> In addition, chromosomal regions with short repetitive DNA sequences are highly variable between individuals and can interfere with CGH analysis.<ref name="Oostlander,Meijer,Ylstra" /> Therefore, repetitive DNA regions like centromeres and telomeres need to be blocked with unlabeled repetitive DNA (e.g. Cot1 DNA) and/or can be omitted from screening.<ref>du Manoir S, Schrock E, Bentz M, Speicher MR, Joos S, Ried T, Lichter P, Cremer T (1995) Quantitative analysis of comparative genomic hybridization. Cytometry 19:27–41.</ref> Furthermore, the resolution of conventional CGH is a major practical problem that limits its clinical applications. Although CGH has proven to be a useful and reliable technique in the research and diagnostics of both cancer and human genetic disorders, the applications involve only gross abnormalities. Because of the limited resolution of metaphase chromosomes, aberrations smaller than 5–10 Mb cannot be detected using conventional CGH.<ref name="Forozan,Karhu,Kononen,Kallioniemi,Kallioniemi" />
For the detection of such abnormalities, a high-resolution technique is required.
Array CGH overcomes many of these limitations. Array CGH is characterized by a high resolution, its major advantage with respect to conventional CGH. The standard resolution varies between 1 and 5 Mb, but can be increased up to approximately 40 kb by supplementing the array with extra clones. However, as in conventional CGH, the main disadvantage of array CGH is its inability to detect aberrations that do not result in copy number changes and is limited in its ability to detect mosaicism.<ref name="Oostlander,Meijer,Ylstra" /> The level of mosaicism that can be detected is dependent on the sensitivity and spatial resolution of the clones. At present, rearrangements present in approximately 50% of the cells is the detection limit. For the detection of such abnormalities, other techniques, such as SKY (Spectral karyotyping) or FISH have to still be used.<ref>{{cite journal | vauthors = Shaw CJ, Stankiewicz P, Bien-Willner G, Bello SC, Shaw CA, Carrera M, Perez Jurado L, Estivill X, Lupski JR | year = 2004 | title = Small marker chromosomes in two patients with segmental aneusomy for proximal 17p | journal = Hum Genet | volume = 115 | issue = 1| pages = 1–7 | pmid = 15098121 | doi = 10.1007/s00439-004-1119-5 | s2cid = 1093845 }}</ref>