Morphological analysis at the neuromuscular junction of hiw^ΔCterm mutants reveals a dramatic synaptic overgrowth phenotype apparent at every type I synapse.

Using the MN4-Ib motoneuron as a model, hiw^ΔCterm mutants exhibit a fivefold increase in the numbers of both synaptic bouton and branch points when compared with wild-type neuromuscular junctions. The synaptic span is also incrase from approximately 15% in wild-type to approximately 40%. Although there is an increase in the number of synaptic boutons, there is an approximate 70% decrease in the average bouton size compared to wild-type.