Dominique Stephens (left), Dinari Harris (center), and colleagues used TIRF microscopy to identify factors associated with secretory vesicles containing the enzyme MMP-9 in breast cancer cells and monitored their dynamics over the course of exocytosis. In the example shown, Rab27b (magenta) colocalizes with MMP-9 (green) at steady state (top images). Both proteins rapidly disperse from exocytic sites following membrane fusion (bottom images).