Casein kinase, a ubiquitous well-conserved protein kinase involved in cell metabolism and differentiation, is characterised by its preference for Ser or Thr in acidic stretches of amino acids. The enzyme is a tetramer of 2 alpha- and 2 beta-subunits. However, some species (e.g., mammals) possess 2 related forms of the alpha-subunit (alpha and alpha'), while others (e.g., fungi) possess 2 related beta-subunits (beta and beta'). The alpha-subunit is the catalytic unit and contains regions characteristic of serine/threonine protein kinases. The beta-subunit is believed to be regulatory, possessing an N-terminal auto-phosphorylation site, an internal acidic domain, and a potential metal-binding motif. The beta subunit contains, in its central section, a cysteine-rich motif, CX(n)C, that could be involved in binding a metal such as zinc. The mammalian beta-subunit gene promoter shares common features with those of other mammalian protein kinases and is closely related to the promoter of the regulatory subunit of cAMP-dependent protein kinase.