Lucifer Yellow is a fabulously-named, highly fluorescent dye used in microbiology experiments to track individual cells. For example, it is used to detect the presence of electrical synapses in a neuron. The neuron is injected with Lucifer Yellow, and if at a future time, neighboring neurons are observed to contain the dye, then electrical synapses must connect the two neurons. Lucifer Yellow's low molecular weight allows it to pass through an electrical synapse's gap junctions.