It's all in the assay: a new model for retinotectal topographic mapping

Ephrin-As have been implicated as topographic mapping labels in the retinotectal system, but the underlying molecular mechanisms for their activities in this context remain somewhat mysterious. Hansen et al. (this issue of Neuron) developed an assay that reveals new mechanisms for ephrins in topographic mapping and suggest a model whereby retinal axons grow and terminate in the tectum via a balance of growth promotion and repulsion, with the balance point depending on retinal position and concentration of ephrin-As.