on the microscopic scale which may lead to the identification of candidate genes to be further explored by other cytological techniques. Through the use of DNA microarrays in conjunction with CGH techniques, the more specific form of array CGH (aCGH) has been developed, allowing for a locus-by-locus measure of CNV with increased resolution as low as 100 kilobases. This improved technique allows for the aetiology of known and unknown conditions to be discovered. History The motivation underlying the development of CGH stemmed from the fact that the available forms of cytogenetic analysis at the time (giemsa banding and FISH) were limited in their potential resolution by the microscopes necessary for interpretation of the results they provided. Furthermore, giemsa banding interpretation has the potential to be ambiguous and therefore has lowered reliability, and both techniques require high labour inputs which limits the loci which may be examined. The first report of CGH analysis was by Kallioniemi and colleagues in 1992 at the University of California, San Francisco, who utilised CGH in the analysis of solid tumors. They achieved this by the direct application of the technique to both breast cancer cell lines and primary bladder tumors in order to establish complete copy number karyotypes for the cells. They were able to identify 16 different regions of amplification, many of which were novel discoveries. Soon after in 1993, du Manoir et al. reported virtually the same methodology. The authors painted a series of individual human chromosomes from a DNA library with two different fluorophores in different proportions to test the technique, and also applied CGH to genomic DNA from patients affected with either Downs syndrome or T-cell prolymphocytic leukemia as well as cells of a renal papillary carcinoma cell line. It was concluded that the fluorescence ratios obtained were accurate and that differences between genomic DNA from different cell types were detectable, and therefore that CGH was a highly useful cytogenetic analysis tool. Initially, the widespread use of CGH technology was difficult, as protocols were not uniform and therefore inconsistencies arose, especially due to uncertainties in the interpretation of data. However, in 1994 a review was published which described an easily understood protocol in detail and the image analysis software was made available commercially, which allowed CGH to be utilised all around the world. As new techniques such as microdissection and degenerate oligonucleotide primed polymerase chain reaction (DOP-PCR) became available for the