Fluorescence in situ hybridization (FISH) is a laboratory technique used to detect and localize specific DNA sequences within cells or tissue samples. It involves using fluorescently labeled probes that bind to complementary DNA sequences, allowing researchers to visualize the presence and location of specific genes or chromosomal abnormalities under a fluorescence microscope. FISH is commonly used in genetics, cancer research, and prenatal diagnostics. It helps identify chromosomal rearrangements, such as deletions or duplications, and can provide valuable information for diagnosing genetic disorders and understanding tumor characteristics in cancer patients.