The TK2 gene has been mapped to chromosome 16q21, where it encodes an enzyme called deoxyribonucleoside kinase. This mitochondrial enzyme comprises of 265 amino acids and weighs approximately 31 kDa. Deoxyribonucleoside kinase enzyme is required for mitochondrial DNA synthesis; it phosphorylates thymidine, deoxycytidine, and deoxyuridine.
Defects in the deoxyribonucleoside kinase enzyme are the cause of mitochondrial DNA depletion syndrome 2, also known as TK2-related mitochondrial DNA depletion syndrome, myopathic form. This is an autosomal recessive disorder due to mitochondrial dysfunction characterized by childhood onset of muscle weakness, and associated with depletion of mtDNA in skeletal muscle.