Casein kinase II (CK2) is a constitutively active, ubiquitously expressed serine/threonine protein kinase that is thought to have a regulatory function in cell proliferation, cell differentiation and apoptosis. CK2 functions as a tetrameric complex consisting of two regulatory beta-subunits and two catalytic units (alpha and alpha') in a homomeric or heteromeric conformation. Whilst the alpha- and alpha'-subunits are catalytically identical, proteins that regulate CK2, such as cdc2 and Hsp90, preferentially bind to the alpha and not the alpha'-subunit. CK2 can phosphorylate a number of key intracellular signaling proteins implicated in tumor suppression (p53 and PTEN) and tumorigenesis (myc, jun, NF-kappaB). CK2 is also thought to influence Wnt signaling via beta-catenin phosphorylation and the PI 3-K signaling pathway via the phosphorylation of Akt.