For decades, “drug discovery” has meant screening millions of compounds to find one that will block a disease process. But Alexis Borisy says that approach is too simplistic. Since “the body always uses mixtures of molecules to regulate itself,” he says, it makes sense to do the same when treating disease. In 2000 Borisy founded CombinatoRx in Boston to search for molecules already proven safe for humans but that combat disease only when used in combination. Along the way, he had to reinvent drug screening. While pharmaceutical companies typically ask only one question about each compound- does it have an effect?- CombinatoRx examines compounds two at a time, in different doses, answering that question 36 separate times. Borisy says the company had to create new lab processes and write software to analyze its masses of data. CombinatoRx has raised $60 million and has launched human trials of three drug combinations designed to treat cancer and rheumatoid arthritis. “Fifty years from now,” Borisy says, “a majority of drugs will be combination by design.”