Fluorescence in situ hybridization (FISH) is a technique that uses fluorescent probes that bind only to parts of a chromosome with a high degree of sequence complementarity. The fluorescence-labeled DNA probes specific to chromosomal regions of interest are used to identify DNA changes on a chromosome. FISH is used to identify numerical abnormalities of chromosomes as well as deletions, duplications, translocations and inversions of certain disease-specific regions. FISH is very useful in identifying deletions, duplications and rearrangements of small disease regions when the presence of these anomalies is unidentifiable by standard chromosome analysis. In hematopoietic neoplasms, FISH aids in diagnosis, prognosis and choice of treatment options.