Rationale: mSec7-1 is a guanine nucleotide exchange factor (GEF). Overexpression of rat msec7-1 in Xenopus neuromuscular junctions causes and increase in the frequency of spontaneous synaptic currents (Fig. 3), without a change in the number of synaptic terminals (data not shown), indicating the the presence of a larger readily releasable pool (Fig. 3). Overexpression of msec7-1 caused an increase in amplitude of the the initial action-potential evoked EPSC. These data indicate a function of msec7-1 in regulating synaptic vesicle exocytosis.. Experimental description: Expression vectors for full length rat msec7-1 were generated