What extracellular signalling mechanisms control adult neurogenesis? In the adult mouse, neurons are formed in two brain regions, the subventricular zone(SVZ) and the dentate gyrus of the hippocampus. In previous studies,Zimmermann and colleagues have shown that stem cells in these regions express NTPDase2, an extracellular nucleotide hydrolyzing enzyme (also called ecto-nucleotidase) that modulates cell communication by nucleotide receptors. They have now taken this further by investigating whether adult neural stem cells (NSCs) respond to extracellular nucleotide signalling itself (see p. 675). The researchers used NTPDase2-expressing neurospheres (clonal aggregates of NSCs)derived from the SVZ. Using Ca2+-imaging, they show that these neurospheres express the nucleotide receptors P2Y1 and P2Y2. The activation of these receptors increased cell proliferation synergistically with epidermal growth factor and fibroblast growth factor 2, and the researchers confirmed this in P2Y1-knockout mice. Although synergism between extracellular nucleotides and growth factors has long been considered important in the brain, this is the first study to demonstrate an effect in adult NSCs.