The PEGylated form of ADI (ADI-PEG20) has shown satisfactory therapeutic results in treating liver cancer and melanoma and has been applied in clinical trials [22, 24], but its therapeutic efficacy is associated with the expression of ASS, it becomes ineffective when the cancer cells are ASS-positive or when the expression of ASS gene is up-regulated during the ADI-PEG20 treatment. The gene discussed is ASS1; the disease is liver cancer.