FISH (fl uorescent in situ hybridization) is a molecular cytogenetic method to detect large-scale genetic alterations in tissue and/or cells. Numerical aberrations (deletions and amplifi cations) and structural aberrations (translocations and fusions) are detectable. Probes bind complementary to the DNA strand of the region of interest. Subsequently, the probes are detected via fl uorochromes and appear as colored dots that can be assessed under the fl uorescence microscope. In situ hybridization is divided into three steps: pretreatment, hybridization, and posthybridization. Pretreatment opens up the cell membranes for hybridization, so that the probe can bind to the complementary DNA target. Posthybridization includes washing steps to remove excessive probes and detection of probes via secondary marked fluorochromes. DAPI stains nuclei and serves as mounting media.