An experiment is described in this test that was designed to study the role of the cyclin B protein in a cell-free system. The work was performed in the lab of Tim Hunt who, together with Hartwell and Nurse, received the Nobel Prize in Physiology or Medicine in 2001 "for their discoveries of key chemicals that regulate the cell division cycle." It was already known at the time of the experiment that the entire genome of eukaryotic cells is replicated once and only once in each cell cycle, and that the cells should pass through an M-phase to be able to replicate their DNA in the next S-phase. A cell-free system containing sperm nuclei and extracts from frog eggs is suitable to study DNA replication. The role of cyclin B was analyzed using this model system in this experiment. (Contains 2 figures and 1 footnote.)