Cyclin Dependent Kinase (CDK) is a family of proteins that are involved in regulating the cell cycle. CDKs are responsible for controlling the progression of the cell cycle by phosphorylating specific proteins, which in turn activate or inhibit other proteins. CDKs are activated by binding to cyclins, which are proteins that are produced in a cyclical pattern throughout the cell cycle. CDKs are also involved in other cellular processes such as transcription, DNA replication, and apoptosis.