The most frequently detected non-diagnostic pathogenic variants were (1) m.1555A>G (NC_012920.1) in MT-RNR1 (10 probands) associated with susceptibility to aminoglycoside ototoxicity, (2) m.3243A>G (NC_012920.1) in MT-TL1 (10 probands) associated with mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), although highly phenotypically heterogeneous, and (3) m.11778G>A (p.(Arg340His), NC_012920.1) in MT-ND4 (7 probands) associated with LHON. The gene discussed is MCAT; the disease is lactic acidosis.