PKA (or cAPK) is a cyclic AMP dependent protein kinase. When activated by the second messenger cAMP, PKA mediates diverse cellular mechanisms, including proliferation, ion transport, regulation of metabolism, plus gene transcription. PKA is comprised of two dimers of two subunits, R (regulatory) and C (catalytic). Two families of R subunit (RI and RII) and three C subunit isoforms (C alpha, C beta, and C gamma) have been identified each possessing distinct cAMP binding properties and resulting in different phosphorylation states. C subunit is activated through autophosphorylation and direct phosphorylation at Thr197 by PDK-1. Tissue specific expression of C gamma, indicates pressure on C gamma during evolution, acting to modulate it in a functionally specific way. Certain amino acid substitutions make C gamma a distinct member of the cAMP dependent subfamily of protein kinases, and suggest that C gamma may be distinct in its protein substrate specificity or its interaction with the different regulatory subunits.