DSCAM and Dravet syndrome: This study shows that expression of a third copy of Dscam, a homologue of one of the genes on HSA21, was sufficient to modify synaptic function and disrupt locomotor performance in the model organism Drosophila. This novel evidence further elucidates the function of DSCAM. Further work is required to fully describe the role(s) of DSCAM overexpression in DS, particularly in the context of concomitant overexpression of other HSA21 genes.