Scientists coated nanospheres of the anti-cancer drug paclitaxel with a peptide shell that shields the drug as it travels through the circulatory system. When the nanosphere reaches a cancerous tumor, enzymes that enable metastasis slice open the shell to release the drug. The targeted delivery allowed them to safely give mice 16 times the maximum tolerated dose of the clinical formulation of paclitaxel and halted the growth of cancerous tumors.