Hoechst dyes bind specifically to the Adenine-Thymine (A-T) regions of DNA. Uncontaminated cell cultures observed under fluorescence microscopy appear as fluorescing nuclei against a negative background; cultures contaminated with mycoplasma (image on the right) appear as brightly fluorescent uniformly shaped filaments surrounding the cells, or as small clusters between the cells. Hoechst DNA staining is the fastest and cheapest method used to diagnose mycoplasma infection.