Aryl hydrocarbon receptors (AHRs) and their co-factor, aryl hydrocarbon receptor nuclear translocator (ARNT), are best known for their role in the mammalian response to environmental toxins. On p. 819, however, Huang and colleagues report that ahr-1 and aha-1, the Caenorhabditis elegans orthologues of AHR and ARNT,specify GABAergic neuron cell fate in nematodes. GABAergic neurons useγ-amino butyric acid as a neuronal transmitter and regulate movements such as foraging behaviours. In C. elegans, the four GABAergic neurons that innervate the head constitute two pairs - RMEL/RMER and RMED/RMEV- classified according to cell lineage and gene expression. ahr-1 is normally expressed in the RMEL/RMER neurons, but in a mutant worm lacking functional AHR-1, the RMEL/RMER neurons become RMED/RMEV-like cells. Conversely, ectopic expression of ahr-1 in RMED/RMEV cells converts them into RMEL/RMER cells. These results demonstrate that ahr-1functions as a cell-type specific determinant, a function that the researchers show requires aha-1.