Abstract: The invention relates generally to a method of treatment for a human genetic disease, such as diseases characterized by unbalanced nucleotide pools, e.g., mitochondrial DNA depletion syndromes, and more specifically, thymidine kinase 2 (TK2) deficiency, using gene therapy. The gene therapy may involve administration of one or more constructs, such as a viral vector, containing a nucleic acid encoding a functional protein. The functional protein may correspond to a nuclear gene. For treatment of TK2 deficiency, the gene therapy may involve administration of one or more constructs, such as a viral vector, containing a nucleic acid encoding a functional TK2 enzyme. The treatment may also involve the administration of pharmacological therapy in conjunction with the gene therapy.