CDK1 also known as cyclin-dependent kinase 1 or cell division cycle protein 2 homolog is a highly conserved protein that functions as a serine/threonine protein kinase, and is a key player in cell cycle regulation. CDK1 is a small protein (approximately 34 kDa), and is highly conserved. When bound to its cyclin partners, CDK1 phosphorylation leads to cell cycle progression. Given its essential role in cell cycle progression, CDK1 is highly regulated. Most obviously, CDK1 is regulated by its binding with its cyclin partners. Cyclin binding alters access to the active site of CDK1, allowing for CDK1 activity. Cyclin B1 contributes to the switch-like all or none behavior of the cell in deciding to commit to mitosis. Cyclin B1-CDK1 is involved in the early events of mitosis, such as chromosome condensation, nuclear envelope breakdown, and spindle pole assembly. The role of cyclin B1 is to transition the cell from G2 to M phase but becomes unregulated in cancer cells where overexpression of cyclin B1 can lead to uncontrolled cell growth by binding to its partner Cdks. Binding of Cdks can lead to phosphorylation of other substrates at inappropriate time and unregulated proliferation.