HQ-O Ready-To-Dilute (RTD^TM) Stain Reagent is designed to label amyloid plaques in paraffin-embedded or freshly cut frozen tissue sections. As a fluorescent zinc chelator, HQ-O is unique as it takes advantage of the known presence of concentrated zinc in amyloid plaques. Studies with HQ-O revealed that fluorescent plaque-like structures are only seen when synthetic Aβx-42 is aggregated in the presence of zinc. Under blue light excitation, plaque structures appear bright green fluorescent in the brain parenchyma, correlating closely with plaque structures observed following Aβ antibody staining. HQ-O RTD^TM staining reagent is compatible with other fluorophores, such as DAPI, Hoechst and ethidium bromide, as well as fluorescent-labelled antibodies with emission spectra in the blue and/or red emission range of fluorescent microscopes. Due to its zinc-chelating characteristics, HQ-O RTD^TM staining reagent may visualize globular structures within blood vessels and intravascular leucocytes. HQ-O RTD^TM staining reagent has multiple advantages over older blue-light exciting stains such as Thioflavin S. Thioflavin S typically exhibits relatively low contrast and resolution and suffers from bleed-through when excited by wavelengths other than blue light. HQ-O RTD^TM staining reagent suffers none of these setbacks and not only provides a higher contrast and longer lasting dye, but because it lacks excitation bleed-through, HQ-O can be readily adapted to multiple labelling studies very easily. To visualize the HQ-O tracer, it is recommended to use a filter cube designed for visualizing Fluorescein/FITC or a blue-light laser. Although it can be seen with both narrow and wide-band pass filters, there is no need to use a narrow band filter since the compound does not bleed through when excited with other filters. A recommended excitation range of a wide band filter is 447-503 nm, with a peak at 475.