The cell cycle is an ordered set of events by which cells undertake growth and division into two daughter cells. In eukaryotes, the cell cycle is described by five phases: G₀, G₁, S, G₂, and M. In G₀ phase, cells have entered a quiescent state or are not dividing. During G₁ period, the cells grow and accumulate nutrients for the subsequent S phase, where DNA replication occurs. The gap between S and M phases is termed G₂ period, during which the cells continue to grow and prepare for mitosis that occurs in M phase. All these processes are elaborately controlled to ensure proper cell development and renewal. The key molecules that program cell cycle are cyclins and cyclin-dependent kinases (CDKs), whereas inhibitors that prevent improper cell division are two families of genes, the Cip / Kip family and the INK4 family. Notably, these inhibitor genes are also termed tumor suppressor genes as they are instrumental in the prevention of tumor formation.