Publisher Summary Casein kinase I and casein kinase II are unique protein kinases that have been described in a number of mammalian and avian cells; an enzyme with properties similar to those of casein kinase I has been described in yeast and plants. The casein kinases prefer acidic substrates and appear to differ from the enzyme endogenous to the mammary gland. Casein kinases I and II are multipotential in the sense that a number of endogenous substrates have been identified for them. Other multipotential protein kinases include the cAMP-dependent and cGMP-dependent protein kinases and phosphorylase kinase. No physiological regulator for casein kinase I has been identified, but the enzyme requires Mg2+ for activity and is stimulated by monovalent cations; the cation requirement is similar for casein kinase II.