View source for Comparative genomic hybridization

== Modifications of CGH ==
Although the classical method allows whole-genome screening without the need for culture and detection of aberrant sites without prior knowledge of their location, it encounters complications in many respects. '''Limitations of conventional CGH''' include the necessary presence of at least 50% aberrant cells in the sample, the inability to detect aberrations without quantitative changes, the differentiation of diploid and tetraploid tumours, and the low resolution of the method  (5–10&nbsp;Mb). 

These shortcomings can be addressed by  '''modification of CGH methods''' (HR-CGH, array CGH). 

=== HR-CGH (''High Resolution CGH'') ===
it differs from classical CGH in the way of software processing of the captured fluorescence image. It uses '''dynamic standard reference intervals''' (natural variation of fluorescence intensities of chromosome pairs) instead of fixed ones. Dynamic values are obtained by statistically processing the deviations of control profiles in individuals with a normal karyotype.

The advantage of the method is the resolution at the level of '''4–5&nbsp;Mb''' and the possibility of detecting aberrations even at '''20–30%''' clonal representaion.

=== ''Array CGH'' ===
Hybridization of fluorescently labelled probes of the DNA under investigation and reference DNA '''with many specific DNA sequences''' (instead of chromosomes). The genome is thus divided into '''small sections and arranged in a grid''' (''microarray'', biochip). The individual sections have a precise location and can thus represent individual genes or parts of genes.

After hybridisation with the DNA under investigation and the reference DNA, the fluorescence ratio for each grid point is calculated using special software. This makes it possible to analyse amplifications or deletions of several thousand genes at once, with a resolution of less than  '''100&nbsp;kb'''. This takes ''array CGH'' to the '''level of molecular diagnostics'''.
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