Control of most bodily functions depends on the ability of cells to talk to each other. We have long known about two routes for cell-to-cell communication: the nervous system and the secretion of hormones. Over the past five years, scientists have recognized an important third route of communication based on exosomes—tiny sacs or vesicles containing protein and RNA molecules that cells secrete into circulation where they can be taken up by other cells to regulate metabolism. Many labs are now focusing on exosomes carrying microRNAs. These are very short RNAs that can regulate the ability of other longer RNAs that make different cellular proteins and control cell function. Thus, microRNAs affect many aspects of cellular behavior in health and disease. Scientists at Joslin Diabetes Center, and collaborators, have now discovered how cells pick a collection of microRNAs for their exosomes, said C. Ronald Kahn, MD, a Joslin Senior Investigator and Professor of Medicine at Harvard Medical School.