fluorescent dye label fiash label This fluorescent label is a small ligand membrane-permeant and nonfluorescent until it binds with high affinity and specificity to the tetracysteine domain. Such in situ labeling adds much less mass than does green fluorescent protein and offers greater versatility in attachment sites as well as potential spectroscopic and chemical properties. MI:0926 4',5'-bis(1,3,2-dithioarsolan-2-yl)fluorescein FIAsH label PSI-MI