Hartwell shared the 2001 Nobel Prize in Physiology and Medicine for discoveries of protein molecules that control the division (duplication) of cells.
In this paper the authors demonstrated a "synthetic model of the cell cycle. Genetics had divided the cycle into two parallel pathways, which comprised two sets of dependent steps and involved a total of 19 cdc genes. At the beginning of the cell cycle, both pathways depend on the completion of a step that Hartwell et al. termed 'start'. This event is defined by the famous cdc28 mutant (famous because the cdc28 gene was later shown to encode the founding member of the cyclin-dependent kinase family), and is also the event at which yeast mating factor arrests cell division to prepare cells for mating. With amazing prescience, the authors speculated that 'start' would turn out to be an important control point for the cell cycle in many eukaryotes" (Patterson, Mark. Nature Reviews Genetics, web focus on cell division, Milestone 5, accessed 7-22).
Order of authorship in the original publication: Hartwell, Culotti, Pringle, Reid.