Biotin-14-dATP is a?dATP analog with biotin attached at the 6-position of the purine base by a 14-atom linker. The long, 14-atom spacer arm allows accessibility to streptavidin for sensitive detection of probe-target hybrids in?in situ?and filter-based hybridizations. Biotin-14-dATP can be efficiently incorporated into DNA by nick translation in the presence of dCTP, dGTP, and dTTP. Biotin-14-dATP can be added to the 3′ end of DNA using terminal deoxynucleotidyl transferase. The amount supplied is sufficient for labeling up to 50 μg of DNA by nick translation.