A 595 nanometer band-pass filter enhances the contrast of in situ hybridization signals on chromosome observed after biotin/avidin-alkaline phosphatase localization.

In situ hybridization with DNA probes labeled with biotin and detected by the avidin-alkaline phosphatase/5-bromo-chloroindoxyl phosphate-nitro blue tetrazolium system has been used to localize DNA sequences in chromosomes. To observe the hybridization signals, a phase contrast microscope has often been used because of the good visibility it provides. Use of a 595 nm band pass filter with the phase contrast microscope enhances signal contrast after in situ hybridization without reducing resolution.