Cyclin H also known as CCNH is a member of the highly conserved cyclin family, Cyclin H plays a crucial role in cell cycle regulation as a component of the cyclin-dependent kinase-activating kinase (CAK) complex, which is essential for the activation of cyclin-dependent kinases (Cdks) that drive cell cycle progression. Cyclin H serves as the regulatory subunit of the CAK, which also includes the catalytic subunit Cdk7. Activation of Cdks, such as Cdk2, is vital for the transition through different phases of the cell cycle, and cyclin H′s interaction with Cdk7 is necessary for the phosphorylation of key threonine residues that enable full enzymatic activity. This phosphorylation is critical, as cyclin H can effectively regulate cell cycle checkpoints, thereby maintaining proper cellular function and preventing uncontrolled cell division, which is a hallmark of cancer.