MB 660R is a bright and photostable far-red dye that emits fluorescence at about 685 nm in the borderline spectral region between far-red and near-IR. Although the absorption maximum is at around 665 nm, this dye can be sufficiently excited by the 633 or 635 nm laser. MB 660R dye is water soluble and pH-insensitive from pH 4 to pH 10. MB 660R is a rhodamine-based dye, and like rhodamine dyes in general, it is exceptionally photostable . The superior photostability and excellent brightness of MB 660R make the dye an ideal choice for confocal microscopy and other demanding applications.