Genetic mosaics of Anabaena filaments show that the hetN ‐dependent paracrine signal propagates from a heterocyst source resulting in the degradation of an activator of differentiation, HetR. HetN and HetR were visualized as fusion proteins to the yellow and cyan fluorescent proteins, respectively. Red autofluorescence outlines cells of the filaments. For further details readers are referred to the article by Rivers et al . on pp. 1260–1271 of this issue