How Cyanobacteria Count to 10

Cell-cell communication is crucial for normal development, and diffusible molecules that regulate differentiation have been identified in several eukaryotic organisms. In this issue [Yoon and Golden][1] show that a diffusible inhibitory peptide, PatS, controls pattern formation in a prokaryotic organism, the cyanobacterium Anabaena . Their results, discussed in a Perspective by Haselkorn, provide insight into the mechanisms underlying heterocyst pattern formation and may apply to differentiation in more complex organisms. [1]: http://www.sciencemag.org/cgi/content/short/282/5390/935