Near-field Scanning Optical Microscopy (NSOM) is a true optical microscopic technique allowing fluorescence, absorption, reflection and polarization contrast with the additional advantage of nanometer lateral resolution, unlimited by diffraction and operation at ambient conditions. NSOM based on metal coated adiabatically tapered fibers, combined with shear force feedback and operated in illumination mode, has proven to be the most powerful NSOM arrangement, because of its true localization of the optical interaction, its various optical contrast possibilities and its sensitivity down to the single molecular level. In this paper applications of `aperture' NSOM to Fluorescence In Situ Hybridization of human metaphase chromosomes are presented, where the localized fluorescence allows to identify specific DNA sequences. All images are accompanied by the simultaneously acquired force image, enabling direct comparison of the optical contrast with the sample topography on nanometer scale, far beyond the diffraction limit. Thus the unique combination of high resolution, specific optical contrast and ambient operation offers many new direction possibilities in biological studies.