FISH technique consist of DNA pieces of interest, tagged with fluorescent dye( probes) , hybridized with patient’s DNA to detect extra or missing pieces in the chromosomes. Easy application, high sensitivith and specificity made this technique the most widely used molecular cytogenetic technique in both research and clinical fields. First double stranded DNA molecule is denatured to the single stranded molecule and then hybridised to flourescently tagged specific probe containing the DNA pieces of interest. The hybrid DNA is visualized under flourescent microscobe to diagnose the missing or extra signals.

FISH is used in prenatal aneuploidy screening, duplications, deletions, translocations and postnatal duplication, deletion, translocation,as well as in cancer genetics; oncogenes, tumour suppressor genes, translocatiıons, inversion or duplication diagnosis.