Clinical epidemiology indicates that there is a high incidence of cancer in patients with type II diabetes. Gene expression profiling experiments predict similarities in regulatory networks common to both type II diabetes and cancer progression. Additionally, there has been some suggestion that patients controlling diabetes by using metformin suffer a lower incidence of cancer diagnosis than patients who do not, suggesting a potential role for metformin in cancer therapy. Standard chemotherapies are not highly effective against cancer stem cells, which may lead to recurrence of tumors. To date, there is little known about treatments that affect cancer stem cells. In the current study, combined metformin and doxyrubicin treatment results in decreased tumor volume in mice and prohibits tumor recurrence for close to three months. Additionally, the authors show that in four different breast cancers, metformin is specifically targeting cancer stem cells. These experiments provide a rationale and experimental basis for using the combination of metformin and chemotherapeutic drugs to improve treatment of patients with breast (and possibly other) cancers. For details, see the article by Hirsch and colleagues on page 7507 of this issue.