A protein long shot

In one race at least, having a chemical advantage over the competition is okay. This race determines which branch from a developing neuron becomes an axon. As Toriyama et al. report on page 147, the winning extension gets a boost from a previously undescribed protein that spurs axon growth.

Although it sports an axon at one end and a fringe of dendrites at the other, a neuron starts out symmetrical. The imbalance develops because the branches, or neurites, that sprout from a youthful cell compete with each other. The fastest-growing extension typically morphs into the axon, and the stragglers become dendrites. Researchers have identified some of the molecular events that dictate which neurite transforms into an axon—the enzyme PI 3-kinase accrues in the winning branch, for example. But they...