Obesity, caused by overgrowth of fat tissue, was shown to be associated with expansion and mobilization of adipose progenitor cells and with increased tumor growth. Animal experiments show that adipose progenitors are recruited by tumors, where they incorporate into the blood vessels and differentiate into adipocytes. A confocal section micrograph of a breast tumor grown in an obese mouse shows vascular/perivascular cells and adipocytes derived from GFP-labeled (green) adipose cells. Vascular endothelium was costained with a CD31 antibody (red), while nuclear staining (blue) identified malignant cells lacking GFP and CD31. For details, see article by Zhang and colleagues on page 5198.