The conserved Aristaless-related homeodomain protein ARX is essential for neuronal development in vertebrates. ARX mutations underlie multiple forms of human X-linked mental retardation and Arx null mice have decreased neural precursor proliferation and defects in GABAergic interneuron differentiation. To determine the precise role of ARX in neuronal development,Melkman and Sengupta are studying the C. elegans homologue of Arx, called alr-1 (see p. 1935). They report that, similar to its Drosophila orthologue, alr-1 acts in a pathway with the LIM1 orthologue lin-11 to specify a subset of chemosensory neurons. In addition, alr-1 is required for the differentiation of a GABAergic motoneuron subtype. These data suggest that,like other homeodomain proteins, some functions of ARX are conserved across species. Consequently, future studies of the simple nervous system of C. elegans may yield insights into the role of ARX in human neuronal development.