The species-specific pattern of lobes or folia in the mammalian cerebellum is largely generated postnatally during the expansion of the granule cell precursor (GCP) pool. Sonic hedgehog (Shh) is a mitogen for GCPs in vitro, and Corrales and co-workers now report that positive Shh signalling through the transcription factor Gli2 is required in vivo to generate enough GCPs for proper foliation by birth (see p. 5581). The researchers show that Shh expression correlates spatially and temporally with foliation, and that in mice lacking Gli2 – the primary activator of Shh signalling – foliation and GCP number in the cerebellum at birth are reduced. Shh overexpression in its normal domain produces normal foliation but increases cerebellar size through increased GCP proliferation. Given their results, the researchers speculate that the evolution of the complex cerebellar foliation patterns of higher mammals may have involved changes in Shh expression patterns.