Neuronal growth cones respond to both contact-mediated and chemotropic
guidance cues; these cues can be either attractive or repulsive. This
past year has seen further characterization of two gene families impl
icated in long-range chemoattraction and chemorepulsion: the netrins a
nd the semaphorins. Analysis of invertebrate members of these gene fam
ilies demonstrates in vivo how netrins play multiple roles in axonal g
uidance in Caenorhabditis elegans, how specific domains of the netrin
molecule confer attractive and repulsive guidance cues, and how semaph
orins can function to generate neuromuscular specificity.