Regulatory subunit of casein kinase II/CK2 (By similarity). 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). Tetramer composed of two alpha chains, one beta chain and one beta' chain. Phosphorylated by alpha subunit. Belongs to the casein kinase 2 subunit beta family.