BP Fluor 405 acid is a water-soluble blue fluorescent dye frequently utilized in multicolor applications, such as flow cytometry and super-resolution microscopy with STORM. It is optimally excited by either the 407 nm spectral line of a krypton laser or a 408 nm violet laser diode. This compound is pH insensitive across the range of pH 4 to pH 10. The carboxylic acid form of BP Fluor 405 serves as an ideal reagent for synthesizing custom active esters not typically available commercially, such as sulfo-NHS, TFP (2,3,5,6-tetrafluorophenol), and STP (4-sulfo-2,3,5,6-tetrafluorophenol, sodium salt). Another prevalent use of non-activated carboxylic acids is in peptide modification during solid-phase synthesis, often requiring in situ activation with peptide coupling reagents like HATU. Additionally, BP Fluor 405 acid is commonly used for control experiments and calibration.