|Cytogenetics is a branch of genetics that is concerned with the study of the structure and function of the cell especially the chromosomes. It includes routine investigation of G-banded chromosomes, other cytogenetic banding methods as well as molecular cytogenetics such as fluorescent in situ hybridization (FISH) and comparative genomic hybridization (CGH).In the event of procedures which allowed very simple enumeration of chromosomes, discoveries were quickly made associated to aberrant chromosomes or chromosome number. In some congenital disorders, such as Down syndrome, cytogenetics disclosed the nature of the chromosomal defect: a "simple" trisomy. Abnormalities originating from nondisjunction events can cause units with aneuploidy (additions or deletions of whole chromosomes) in one of the parents or in the fetus. Methods of cytogenetic banding are Karyotyping, Slide groundwork, Fluorescent in situ hybridization. Analysis of FISH specimens is done by fluorescence microscopy by a clinical laboratory specialist in cytogenetics. For oncology generally a large number of interphase cells are scored in order to rule out low-level residual disease, generally between 200 and 1,000 cells are counted and scored. For congenital problems usually 20 metaphase cells are scored. The future prospects cytogenetics is the improvement on molecular cytogenetics including automated schemes for counting the outcomes of benchmark FISH groundworks and methods for virtual karyotyping, such as relative genomic hybridization arrays, CGH and lone nucleotide polymorphism arrays.