Mitochondrial DNA depletion syndrome (MDS) has been associated with nine nuclear genes involved in the maintenance of mitochondrial dNTP pools, including thymidine kinase 2 (TK2), deoxyguanosine kinase (DGOUK), p53 dependent ribonucleotide reductase subunit 2 (RRM2B) and thymidine phosphorylase (TYMP); the most severe forms of the disease lead to infantile death (for review, see [7,8]). Here, TK2 is linked to mitochondrial DNA depletion syndrome.