Among women with breast cancer, approximately 25% have a subtype that is characterized by high levels of expression of the protein HER2. HER2-positive breast cancer tends to be more aggressive than other breast cancer subtypes. Increased molecular understanding of why HER2-positive breast cancer is so aggressive could aid in the development of new therapeutics. By studying mouse models, a team of researchers, led by Lewis Chodosh, at the University of Pennsylvania School of Medicine, Philadelphia, has now identified a key role for the protein Hunk in the formation of tumors driven by overexpression of HER2. As the data also indicate that Hunk is required for HER2-induced tumor cells to maintain their cancerous characteristics, the team suggests that inhibiting Hunk might provide a new approach to treating HER2-positive breast cancer.