Fat cells (adipocytes) surround breast tumors and contribute to tumor growth by expressing factors that aid oncogenesis. Col6 is a protein that is highly expressed in adipocytes and its expression is further increased in both obesity and in breast cancer cells. In this issue of the Journal of Clinical Investigation, Phillip Scherer and Jiyoung Park of the University of Texas Southwestern Medical Center report that a portion of the Col6 protein, known as endotrophin, alters the tumor environment to promote tumor growth and metastasis in mice. Mice with reduced endotrophin expression had significantly less tumor burden and fewer metastases. These findings link obesity and cancer progression and suggest that endotrophin may serve as novel therapeutic target in the treatment of breast cancer.