Cancer that has spread (metastasized) to the brain is normally incurable because the protective blood-brain barrier blocks most treatments from entering. Nanoparticles capable of transporting drugs and crossing the barrier can help deliver life-saving therapies to these tumors. This image shows blood vessels (red), cell nuclei (blue) and metastatic human breast cancer cells (green) in the brain of a mouse, after intravenous administration of experimental nanoparticles capable of crossing the blood-brain barrier.