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 (By similarity). Casein kinase II/CK2 is a tetramer composed of two alpha subunits 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.