MAT1A and hepatocellular carcinoma: Further studies demonstrated the down-regulation of the key enzymes of the methionine cycle, including MAT1A, GNMT, BHMT, CBS, and methionine synthetase (MS) in human cirrhosis and HCC [56], which may explain the hypermethioninemia, the hyperhomocysteinemia, and the decreased hepatic GSH levels observed in cirrhosis.