The proper functioning of the nervous system depends on precise interconnections of distinct types of neurons. Therefore, understanding how neurons specifically find and recognize their target cells is a central question in neuroscience. Involvement of specific molecular cues expressed on target cells has long been suggested. However, the nature of such cues remains poorly understood. To comprehensively address this problem, collaborating with Prof. Hiroyuki Aburatani (Research Center of Advanced Science and Technology, University of Tokyo), we conducted single-cell microarray analysis of target muscles in Drosophila (the insect fruitfly), which are innervated by distinct motor neurons. By this new method, we identified a secreted protein, Wnt4, as a novel target cue that regulates synaptic specificity in a negative manner. The results provide evidence that synaptic target specificity is generated by a repulsive signal coming from non-target cells.