Accumulating evidence indicates that in addition to the classical complex, the catalytic and regulatory subunits of CK2 can also exist as free populations in living cells. The association of recombinant CK2 subunits in vitro has been characterized, providing evidence for the first time for their targeted interactions in living cells. The data also suggest that the regulation by phosphorylation of many CK2 substrates may strongly depend on the dynamic localization/association of its subunits.