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Neurobiologist and Nobel Laureate R. W. Sperry proposed an influential chemo affinity theory half a century ago (1) to explain the precision of neuronal wiring in the brain: “The cells and fibers of the brain must carry some kind of individual identification tags, presumably cytochemical in nature, by which they are distinguished one from another almost, in many regions, to the level of the single neurons” (1). He suggested that gradients of such identification tags on retinal neurons and on the target cells in the brain coordinately guide the orderly projection of millions of developing retinal axons. This idea was supported by the identification and genetic analysis of axon guidance molecules, including those that direct development of the vertebrate visual system (2). But axons not only perceive molecular cues on their targets; they also recognize those on fellow axons. On page 585 of this issue, Imai et al. provide a striking example of how axon-axon interactions organize olfactory axons before they reach their target in the brain (3).