Adenylate kinases (AKs) are nucleoside monophosphate kinases, which catalyze the phosphorylation of AMP by using ATP or GTP as phosphate donors. AKs are thus involved in maintaining the homeostasis of cellular nucleotides. CMP, dCMP and dAMP are other substrates phosphorylated with less efficiency by AKs. Cytosolic adenylate kinases 5, 7, 8 and 9 (AK5, 7, 8 and 9) catalyze the reversible phosphorylation of (d)AMP and (d)CMP with ATP to form (d)ADP and (d)CDP, respectively, and ADP. When GTP is the phosphate donor, only AMP and CMP are efficiently phosphorylated (Panayiotou et al. 2011, Amiri et al. 2013). In the body AK5 expression was observed only in brain of nine tissues tested by Northern blotting (Van Rompay et al. 1999). AK5 is inferred to occur as a dimer from unpublished crystallographic data obtained for the catalytically active carboxyterminal third of the protein (PDB 2BWJ).