IntroductionHere we describe fluorescence in situ hybridization (FISH) of DNA probes to histological sections, which allows the visualization of specific DNA targets (chromosome territories and their subregions) in the context of functional tissue organization. Separate protocols are provided for hybridization using paraffin-embedded tissue sections and for hybridization using vibratome or frozen sections. Pretreatment with heat or protease is necessary to allow unmasking of the target DNA and efficient penetration of reagents in the nuclei. Because the goal of the technique is to obtain data on the native 3D structure of the genome, close attention is paid to the preservation of nuclear morphology.