Many molecules that promote neuronal and axonal growth cone migrations during neuronal development have been identified. Now, however, Vanderzalm and colleagues report that CRML-1 (the C. elegans homologue of the mammalian actin-uncapping protein CARMIL) negatively regulates these important migrations in C. elegans by inhibiting Rac signalling (see p. 1201). Signalling through Rac GTPases is known to be involved in axonal migration because mutations in Rac GTPases and in their guanine nucleotide exchange factors(GEFs) disrupt axonal migration in Drosophila and C. elegans. The researchers identify CRML-1 as a negative regulator of neuron and axon migrations in a C. elegans genetic screen. They then show that CRML-1 inhibits the Rac GEF activity of UNC-73 (a homologue of the mammalian Rac and Rho GEF Trio) and that CRML-1 lowers the levels of the neuronal guidance receptor SAX-3; UNC-73 increases the levels of this Robo homologue. Together, these results reveal a novel role for a CARMIL homologue- the negative regulation of neuron and axon migrations through the inhibition of Rac signalling.