Molecular cytogenetics began in 1977 with in situ hybridization of DNA probes to human chromosomes method (FISH), which circumvented some limitations of tradicional cytogenetics. A new method of analysis, array-based comparative hybridization (array CGH) was increasingly being used in clinical practice and pro - vided the opportunity to reveal imbalances across the whole genome. Array CGH is recently developed and rapi dly progressing technology of molecular cytogenetics, which evidently erased the landmarks between cytogenetics and molecular genetics, enhances our understanding of the complexity of the human genome, is attempting to change the schemes of patient’s routine investigation and spreads in all areas of medicine. This new diagnostic technique was suc - cessfully applied to the diagnostic investigation of mental retardation. Diagnostic yield of this analysis is approximately 10% of cases, in which diagnosis was unknown after conventional caryotyping. The use of array CGH has expanded our knowledge of phenotypic spectrum of recognisible syndromes, revealed new microdeletion syndromes and genetic causes of several known conditions. Since array CGH does not require the stimulation of cell cultures by phytohemagglutinin, which may distort the proportion of mosaic cells and inhibit the detection of mosaic cases, detection of mosaicism increased up to 8-10% using array CGH method in cases of menial retardation, development delay or dysmorphic features. Array CGH technique was also successful in revealing the molecular mechanisms of some chromosomal aberrations and identification of reciprocal products of known conditions.
