technique that employs fluorescent probes for the detection of specific deoxyribonucleic acid ( DNA) sequences in chromosomes (chromosome). FISH has a much higher rate of sensitivity and specificity than other genetic diagnostic tests such as karyotyping and thus can be used to detect a variety of structural abnormalities in chromosomes, including small genetic deletions involving just one to five genes (gene). It is also useful in detecting moderate-sized deletions such as those causing Prader-Willi syndrome, a rare genetic disorder characterized by a rounded face, low forehead, and intellectual disability. FISH also provides results more quickly than karyotyping because no cell culture is required.

FISH is commonly used for preimplantation genetic diagnosis (PGD) during in vitro fertilization. PGD involves obtaining a single cell from an embryo in the blastocyst stage of development. This single cell can then be analyzed using FISH. One problem with using FISH for PGD is that a single cell is scant material for diagnosis; therefore, a large array of tests cannot be performed. Similarly, if the test fails for any technical reason, it cannot be repeated.