Most cellular effects of cAMP are mediated through PKA by serine or threonine phosphorylation of target proteins. Human Protein Kinase A (cAMP Ser/Thr Protein Kinase Family) is an inactive tetrameric cytoplasmic holoenzyme composed of homo- or heterodimeric regulatory subunits associated with two catalytic subunits. cAMP causes dissociation of inactive holoenzyme into a cAMP-bound dimer of regulatory subunits and two free monomeric catalytic subunits that phosphorylate many cytoplasmic and nuclear substrates. In human, regulatory PRKAR1A, PRKAR1B, PRKAR2A, and PRKAR2B subunits and catalytic PRKACA, PRKACB, and PRKACG subunits have been identified. (NCI) [ ]