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 genetic research, clinical diagnostics, and cancer studies. It helps identify chromosomal rearrangements, gene amplifications, and deletions, providing valuable information for understanding genetic disorders and guiding treatment decisions in conditions like cancer and genetic syndromes.