Adult stem cells are regulated by interactions with neighbouring differentiated cells, but what is the molecular basis of these interactions?Such information would be useful in the context of regenerative medicine and cancer biology. Now, Oviedo and Levin report that, in the planarian worm Schmidtea mediterranea, the regulation of neoblasts (adult stem cells that proliferate after injury and regenerate damaged tissues) requires smedinx-11, which encodes an innexin, an invertebrate gap junction protein (see p. 3121). The researchers investigated innexin transcripts in S. mediterranea as potential regulators of neoblasts because gap junction-permeable signals have been implicated in embryonic patterning and morphogenesis. They show that smedinx-11 is expressed in the neoblasts and that treatment of the worms with smedinx-11 RNAi abrogates neoblast proliferation and inhibits regeneration. It also prevents neoblast maintenance and disrupts the normal anterior-posterior gradient of mitotic neoblasts. The researchers suggest, therefore, that gap junctional communication regulates the interactions of adult stem cells with differentiated cells that control their behaviour in multicellular organisms.