Protein kinase A (PKA) has two regulatory subunits and two catalytic subunits which are held together to form the holoenzyme and is activated upon binding of cAMP to the regulatory subunits. Once cAMP binds the regulatory subunits, the catalytic subunits are released to carry out phosphorylation of CREB1 at serine residue S133. Only the PKA catalytic subunit alpha, PRKACA, was directly demonstrated to phosphorylate CREB1 at S133, using recombinant mouse and rat proteins, respectively (Gonzalez and Montminy 1989). PKA catalytic subunits beta and gamma (PRKACB and PRKACG) are candidate CREB1 kinases based on indirect evidence and sequence similarity (Nagakura et al. 2002, Liang et al. 2007, James et al. 2009). PRKX is the catalytic subunit of the cAMP dependent protein kinase X, which shares the regulatory subunits and functional properties with the PKA. PRKX is highly expressed in the mouse fetal brain (Li et al. 2005) and is implicated in CREB1 phosphorylation through indirect evidence (Di Pasquale and Stacey 1998, Li et al. 2002).