Cancer can be described as improper cell proliferation. To understand cancer it is necessary to understand the genetic and biochemical mechanisms responsible for proper cell proliferation. Since the early 1990s, several families of novel proteins involved in cell-cycle regulation, and the clarification of the biochemical process in which they have participated have been classified. This text discusses the roles of various cell-cycle regulators - cyclin, cyclin-dependent kinases (CDKs) and CDK inhibitors - and describes the connections between these proteins and oncogenesis. Possible ways of clinical intervention that might be developed into potent cancer therapies are also explored.