In the phase 1 trial, 27 patients were enrolled (23 with advanced HCC and 4 with other GI tumors). No dose-limiting toxicities were observed in cohort 1 or 2, and the recommended phase 2 dose for ADI-PEG 20 was 36 mg/m2 weekly with mFOLFOX6. The ORR was 21% (95% CI 7.5–43.7), with a median PFS and OS of 7.3 and 14.5 months, respectively. Arginine levels were depleted despite low levels of anti-ADI-PEG 20 antibodies, but there was no correlation between arginine depletion at 4 and 8 weeks and archival tumoral ASS1 levels with response. The gene discussed is ASS1; the disease is digestive system neoplasm.