In addition, other rarer mutationsthat affect mtDNA instability, such as DNA polymerase subunit gamma-1(POLG), thymidine kinase 2 (TK2) or deoxyguanosine kinase(DGUOK), can cause ALS-like symptoms [23, 24, 25]. The gene discussed is TK2; the disease is amyotrophic lateral sclerosis.