Protein kinase A (PKA, cAMP-dependent protein kinase) is a key element of a ubiquitous signaling pathway important in the cell cycle, cellular communication, memory formation and behavior. PKA is composed of two catalytic (PKAc; Protein Kinase A catalytic subunit) and two regulatory subunits (PKAr). Upon binding cAMP, the complex dissociates to PKAr dimer and two activated PKAc ser/thr protein kinase catalytic monomers. The released PKAc can translocate into the nucleus and exert a regulatory role in the activation of multiple nuclear hormone receptors. However, PKAc-mediated activation of tonicity-dependent gene expression is cAMP independent. Humans express three types of PKAc subunit - PKAc alpha is present in most human tissues, PKAc beta and gamma are tissue-specific, the later is found in testes.