There is enormous diversity in the morphology and functional properties of synapses in the nervous system; however, we know little about the molecular pathways that direct the development of particular synaptic specializations. The authors show that expression of the transcription factor MAFB in spiral ganglion neurons (SGNs) is critical for the final stages of differentiation of the highly specialized ribbon synapses that form between cochlear hair cells and SGNs. They further show that MAFB acts downstream of the transcription factor GATA3 and that restoration of MAFB levels is able to rescue synapse defects that result from Gata3 mutations.