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. When GTP is the phosphate donor, only AMP and CMP are efficiently phosphorylated. Adenylate kinase 4 (AK4) can mediate nucleotide homeostasis in the mitochondrion (Panayiotou et al. 2010).