Fecal microbiota from adult human twins–one obese and one not–were transplanted into germ-free mice fed low-fat mouse chow or diets representing typical U.S. menus. The obese phenotype was transmitted. And notably, mice with the lean twin’s microbiota prevented the increased body mass in cagemates harboring an obese twin’s microbiota. The diet changed the outcome. Read more of this fascinating result in Science.