Results of an ongoing phase I clinical trial reinforce the promise of an experimental immunotherapy drug against melanoma. The drug, lambrolizumab, blocks a protein called PD-1, which lets cancer cells evade the immune system. In the new study, researchers gave lambrolizumab to 135 people with melanomas that had spread. Tumors shrank in more than half of those who got the highest dose of lambrolizumab (10 mg/kg every 2 wk), and disappeared completely in 12 people. Moreover, tumors still had not grown again more than a year later. Side effects were generally mild. This trial is currently accepting new participants.