Cy3B maleimide is a mono-reactive dye containing a maleimide group, designed to selectively and efficiently attach the Cyanine3 fluorophore (an analog of Cy3) to proteins and peptides with cysteine residues, as well as other thiolated molecules, such as thiol-containing oligonucleotides. Cy3B is an enhanced version of the Cy3 dye, offering significantly improved fluorescence quantum yield and photostability.