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DiI (DiIC18(3); 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine) is an orange-red fluorescent carbocyanine dye.

DiI is a widely used lipophilic dye that labels cell membranes by inserting its two long hydrocarbon (C18 carbon) chains into the lipid bilayer. The dye is weakly fluorescent until incorporated into membranes. DiI diffuses laterally to stain the entire cell, allowing it to be used as an anterograde and retrograde tracer of neurons. In intact tissue, the dye does not transfer from labeled to unlabeled cells, but some transfer may occur when the membrane is disrupted, for example, after sectioning.

DiI is often used with other tracers in dual-color studies, such as DiA and DiO.