The casein kinase I (CKI) family of serine/threonine protein kinases is highly conserved from yeast to humans. The CKI family is involved in many diverse and important cellular functions, such as regulation of membrane transport, cell division, DNA repair, circadian rhythms, and nuclear localization. The name of the enzyme family was originated from the convenience of casein as a substrate since the earliest days of research on protein phosphorylation. Casein kinase 2 (CK2) is a ubiquitous, highly conserved, essential serine/threonine kinase which has been implicated in cell cycle control, DNA repair, regulation of the circadian rhythm and other cellular processes. CK2 is a tetramer of two α subunits and two β subunits. The α subunits have the catalytic kinase domain. Loss of the α‘ catalytic subunit produces male infertility, and loss of the single β regulatory subunit is early embryonic lethal. CK2 appears to be upregulated in most cancers and the promotion of tumorigenesis by the overexpression of CK2 has been reported in transgenic mice. Casein kinases are operationally defined by their preferential utilization of acidic proteins such as caseins as substrates. The α and α' chains contain the catalytic site. Participates in Wnt signaling. CK2 phosphorylates 'Ser-392' of p53/TP53 following UV irradiation.