The boutons of rtGEF¹ third instar larvae are less defined than in wild type. The subsynaptic reticulum is completely absent at the NMJ between muscles 6 and 7 in rtGEF¹/rtGEF² mutants, but the presynaptic terminal appears to be no different from wild type. However, rtGEF¹/Df(2L)PJ19 mutants exhibit presynaptic defects; there is an excess of vesicles in the terminal, which are larger than those of wild type, although vesicles in the mutant are able to dock at T bars. Evoked excitatory junctional potentials (EJPs) are reduced and there is a greater variation in EJP amplitude per muscle in rtGEF¹/Df(2L)PJ19 mutants compared to wild type. While the amplitude of miniature EJPs is somewhat reduced in mutants, there is a large reduction in their frequency, indicating a presynaptic defect.