When cells divide, they divide in phases called G1 (growth), S (synthesis), G2 (growth), and M (mitosis). Nurse, Hartwell and Hunt together discovered two proteins, cyclin and CDK (cyclin dependent kinase), which control the change from one stage to another. These proteins are called "checkpoints", because they check whether the cell has divided properly. If the cell does not divide correctly, other proteins will try to repair it. If that does not work, they will destroy the cell. If a cell divides incorrectly and survives, it may cause cancer and other serious diseases.