Aminooxy CF640R (aka CF640R hydroxylamine or CF640R aminooxyacetamide) is our superior far red fluorescent dye CF640R with an aminooxy reactive group, which can be used to label a target molecule with an aldehyde or ketone group. Aminooxy group readily reacts with an aldehyde or ketone group to form a stable linkage without the use of a reducing agent. CF640R is a novel rhodamine-based dye spectrally very similar to Cy?5 and Alexa Fluor? 647. CF640R is much brighter than Cy?5 and at least as bright as Alexa Fluor? 647 when excited at 633 or 635 nm. However, the most important advantage of CF640R is its exceptional photostability. CF640R is far more photostable than the cyanine-based dyes Cy?5 and Alexa Fluor? 647. It is also more photostable than Atto 647N, a dye that has been extensively used in single molecule-based imaging. CF640R is also extremely water-soluble and pH-insensitive. These properties make CF640R a much better alternative to Cy?5, Alexa Fluor? 647 and other spectrally similar dyes.