Nature: The human immune system poses one of the biggest challenges to the delivery of drugs to diseased areas of the body because its macrophages target and destroy foreign molecules. Now Dennis Discher of the University of Pennsylvania and his colleagues believe they have found a solution—attaching “safe” peptides to the drug-delivery molecules. Discher’s team used computer simulations to design a small, 21-amino-acid peptide based on the structure of CD47, a membrane protein that macrophages recognize as safe. They then attached the peptide to dye-carrying nanobeads of polystyrene and injected them into mice, along with an equal number of beads not containing the peptide. Using fluorescent spectroscopy to compare their movement through the mice’s bloodstream, the researchers found that four times as many of the altered nanobeads got past the macrophage defenses. They also determined that the nanoparticles tended to cluster in tumors. Adding the anticancer drug paclitaxel to the nanobeads shrank the tumors just as effectively as the traditional paclitaxel delivery system, Cremophor, but without the side effects.