fluorescein label fluorescein-5-maleimide label Fluorescein-5-maleimide is one of the most popular fluorescent dyes for thiol modifications of proteins at cysteine residues that either are intrinsically present or result from reduction of cystines. Unlike iodoacetamides, maleimides do not react with histidines and methionines under physiological conditions. PSI-MI MI:0940 fluor-5-maleimide