The vast majority of top-selling drugs are biologics—also known as proteins. Proteins are used today to treat many debilitating diseases, including arthritis, Crohn’s disease, and several forms of cancer. They have helped to improve the lives of many millions of people worldwide. And proteins have the potential to help many millions more, but they can’t, because most are unable to pass through the cell boundary to reach the regions of the cell where they are needed: the cell interior.