Unlike most serine/threonine kinases, Casein Kinases (CKs) are not activated by specific stimuli, but are essentially constitutively active. CK1 autophosphorylates, causing inactivation, and protein phosphatases reactivate the kinase. The substrates of CKs are numerous, but their function is less clear, perhaps due to the absence of obvious regulation. CK1 isoforms are thought to be involved in the response to DNA damage, while CK2 levels are elevated in many types of cancer.