Regulatory subunit of casein kinase II/CK2. As part of the kinase complex regulates the basal catalytic activity of the alpha subunit a constitutively active serine/threonine-protein kinase that phosphorylates a large number of substrates containing acidic residues C-terminal to the phosphorylated serine or threonine (By similarity). Participates in Wnt signaling. Casein kinase II/CK2 is a tetramer composed of an alpha subunit, an alpha' subunit and two beta subunits. The beta subunit dimerization is mediated by zinc ions. Phosphorylated by alpha subunit. Belongs to the casein kinase 2 subunit beta family.