The role of the mitochondrial l-lactate metabolism merits further focus: given that hydrogen peroxide production in the tumor microenvironment fuels the anabolic growth of cancer cells (28), a possible role of the putative mitochondrial l-lactate oxidase (LOX) which generates hydrogen peroxide in rat liver mitochondria (29) should be investigated; the LOX existence in Hep G2-M appears to be consistent with the evidence that rotenone, which blocks oxygen consumption induced by the addition of malate + glutamate fails to inhibit oxygen consumption induced by the addition of l-lactate. The gene discussed is LOX; the disease is cancer.