Although no obvious changes were observed in the NK cells of physiologic UCP1−/− mice (8–10 weeks old), impaired NK cell bioactivity was shown in methionine–choline-diet (MCD)-fed UCP1−/− mice and involved in the acerbation of nonalcoholic steatohepatitis (NASH) progress to liver fibrosis. This evidence concerns the gene UCP1 and metabolic dysfunction-associated steatohepatitis.