A cancer drug discovered in 2001 that targets the cell cycle has recently shown promising efficacy in metastatic breast cancer. Pfizer's palbociclib, added to standard therapy, doubles the time women live with no new tumor growth. The drug, which inhibits a cell cycle protein called CDK4, has erased the legacy of failure of other cell cycle cancer therapies, and competitors have introduced their own versions, which show promise in multiple tumor types. But the drug could have emerged much sooner, say palbociclib's inventors, had Pfizer used a more sophisticated development strategy and not shelved the drug for much of the last decade. Pfizer disagrees, but it was only after outside investigators demonstrated palbociclib's potential in breast cancer that the company moved the drug forward.