Chromosomal abnormalities are leading cause of genetic diseases, including congenital disorders and acquired diseases. Cytogenetic analysis of numerical and structural chromosomes aberrations in pre and postnatal diagnoses and cancer research is an integral part of clinical care. Until recently, karyotyping was the most conclusive test for detection of chromosome aberration. However, in addition to routine chromosome analysis, fluorescent in situ hybridization technique (better known as FISH) are used to identify various subtle chromosomal changes that are not possible to detect by conventional banded techniques.

FISH is highly sensitive method that enables visual identification of chromosomes or part of chromosomes on slides. This molecular cytogenetics method is based on specific hybridization of the target DNA fixed on slide and probe DNA that has been labeled with a detectable molecule, such as a fluorescent tag. FISH has been used as an addition to routine cytogenetics analysis to achieve a higher sensitivity, specificity and resolution of chromosomes than is possible by karyotyping. The advantages such as speed, sensitivity, high resolution and reliability, make FISH wide spread applicable in clinical cytogenetics, prenatal diagnosis, diagnosis of infectious diseases and tumor biology. In recent years the possibilities for visualizing several chromosomal targets simultaneously has lead to increasing role that FISH plays in the study of complex chromosome rearrangements.

FISH allows specific nucleic acid sequences to be detected in morphologically preserved chromosomes, cells or whole-tissue sections. The process of FISH in most cases is performed on chromosomes that are in metaphase, although it can be used on interphase cells derived from standard cytogenetic preparations, as well as, archived embedded histological material and fixed cytological preparations.