Fusion. If you have ever tried to merge soap bubbles, you know the paradox. Soap bubbles look soft and fragile, but the moment you bring them close, they push each other away. Membranes behave similarly. A synaptic vesicle and the presynaptic membrane do not naturally want to merge. They repel each other. Therefore, synaptic vesicle release requires a mechanism that overcomes this resistance, holds two membranes in close contact, and then triggers fusion on demand within milliseconds. In a recent paper, that mechanism is brought into focus through UNC13A, a gene encoding one of the central priming factors that makes synaptic fusion possible. Here is what the study shows.