The discovery of the Philadelphia chromosome as a hallmark of chronic myelogenous leukemia in 1960 by Peter Nowell provided evidence for a genetic link to cancer. As with most seminal scientific observations, the description of the Philadelphia chromosome posed many more questions than were answered. This Review series includes contributions from individuals who performed critical experiments addressing some of the most important of these questions, reflecting the nearly 50 years of work inspired by Nowell's initial finding. The legacy of the Philadelphia chromosome now serves as a paradigm for how basic science discoveries can lead to effective new approaches for the treatment of human disease.