Early events in neuronal differentiation are generally considered to be regulated by factors independent of alterations in membrane permeability. Weaver mice harbour a mutation that blocks neuronal differentiation just after cessation of cell division, prior to cell migration and synaptogenesis. Cerebellar granule cells in homozygous weaver mice fail to differentiate, either because intrinsic cues are absent or because the granule cells are unable to respond to those cues. We now report that weaver mice have a missense mutation in a gene encoding a G–protein coupled inward rectifier potassium channel. The mutation alters the putative ion–permeable, pore–forming domain of the protein, suggesting that granule cell differentiation is regulated by changes in membrane permeability. © 1995 Nature Publishing Group.