The highly fluorescent label 4-acetylamino-1,8-naphthalimide-N-caproic acid has two fused benzene rings containing a closed anhydride ring and therefore is a flat and planar molecule which can easily intercalate or act as a groove binder with DNA/RNA sequences. For the study of in situ hydridization with complementary synthetic oligonucleotides (ODNs), tagging with non-radioactive labels has gained importance, not only for medical diagnostics or clinical genetics, but also detection and isolation of DNA for research purposes. The pentamethylene chain in 6 acts as a linker molecule and keeps the target at a reasonable distance to avoid direct interaction with oligonucleotides.