Having demonstrated that the PEGylation of ADI did not significantly alter the strong antitumour efficacy compared to that of native ADI in ASS1-deficient human melanoma and hepatocellular carcinoma (HCC) cell lines, Ensor et al. [29] explored how PEGylation optimised drug activity in vivo. The gene discussed is ASS1; the disease is hepatocellular carcinoma.