Interestingly, each of the MTTK variants, including the novel m.8315A>C variant, are localized in different secondary structures of mitochondrial tRNA-Lys (m.8315—linker of D-stem and AC-stem, m.8344—T-loop, and m.8363—acceptor stem), which results in the same pattern of combined OXPHOS complex deficiency (diminished CIV levels, with a profound decrease in CI and CV, and accumulated CV sub-complexes), which is distinct from the MTTL1 gene variant [19]. Here, MT-TK is linked to hyperinsulinemic hypoglycemia, familial, 4.