A new class of nanoparticles that accumulate inside tumors could one day improve imaging quality and cancer treatment by delivering image-enhancing agents or cancer drugs directly to tumor sites. A team led by Erkki Ruoslahti, a professor at the Burnham Institute for Medical Research in La Jolla, CA, coated iron oxide nanoparticles with a peptide that is attracted to protein clots in tumor blood vessels. When injected into mice with breast cancer, the nanoparticles sought out the tumors and bound to their blood-vessel walls. For reasons the researchers do not yet understand, the particles also induced more clotting, which attracted more particles, enhancing their effectiveness and potentially choking off a tumor’s lifeblood. The team is working to ensure that the particles won’t build up in normal tissues.