DiI (DiIC18(3); 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine) is a widely used carbocyanine membrane dye that labels cell membranes by inserting its two long (C18 carbon) hydrocarbon chains into the lipid bilayer. Particularly, it has been extensively used for the anterograde and retrograde labeling of neurons.