A small protein detectable in urine can predict a breast cancer’s aggressiveness, and possibly provide a new avenue for treating the disease. When its production is shut down, cell migration — a key step in progression and metastasis — is inhibited. The protein, which has been licensed for clinical development, also plays a key biological role in advancing cancer, triggering the epithelial-to-mesenchymal transition and reducing cells’ response to the hormone estrogen.