The GRASP assay for detecting synapse formation between specific neurons. Top: the two halves of split GFP are localized to pre- and postsynaptic specializations, respectively. Only when a synapse is formed will the complementary parts of GFP come close enough to reconstitute a fluorescent molecule (left). Bottom: GRASP used as a readout for synapse formation between the PHB and AVA neurons. Fluorescent puncta are only detected when synapses are formed between PHB and AVA (left).