5'3'-Nucleotidase, mitochondrial (NT5M) is the major nucleotidase of human mitochondria, catalyzing the hydrolysis of TMP, uridine 2'-, 3'-, and 5'-monophosphates, and dUMP to yield the corresponding (deoxy)nucleosides and orthophosphate. It may play a central role in "substrate cycles" to regulate mitochondrial deoxynucleotide levels, especially in non-dividing cells (Rampazzo et al. 2000; Gallinaro et al. 2002). The active form of the enzyme is a homodimer, with an absolute requirement for Mg++ (Rampazzo et al. 2000; Rinaldo-Matthis et al. 2002).