While at Edinburgh Sir Paul worked with Professor Murdoch Mitchison and others, using genetics to study the cell cycle responsible for the reproduction of cells, and in particular the processes regulating the cell cycle and the onset of mitosis.
He further developed his research at the University of Sussex, the Imperial Cancer Research Fund (ICRF) labs in London and the University of Oxford, returning to London as Scientific Director to ICRF in 1993. ICRF is now part of the major medical research charity Cancer Research UK.
Sir Paul was awarded the Nobel Prize in Medicine in 2001, along with Leland Hartwell and Tim Hunt, for their discoveries of "key regulators of the cell cycle".