The 13q deletion FISH (Fluorescence in situ Hybridization) technique uses fluorescently labeled DNA probes to detect specific deletion regions, typically at 13q14, on chromosome 13q. This is done by hybridizing these probes to denatured DNA from a patient's cells (often bone marrow or blood for conditions like chronic lymphocytic leukemia (CLL) or multiple myeloma) and analyzing the fluorescent signals under a microscope. The presence of one or two normal signal areas versus a lack of signal indicates a deletion, providing crucial prognostic information for certain cancers