Blood samples (n = 15) from subjects (n = 7) with disorders causing mtDNA depletion were studied, including three patients with polymerase-γ deficiency, and one each with a deficiency of deoxyguanosine kinase, thymidine kinase 2 and ribonucleotide reductase M2B. This evidence concerns the gene TK2 and hyperinsulinemic hypoglycemia, familial, 4.