In conclusion, we have demonstrated here for the first time that: 1) physiological levels of the arginine precursor citrulline are not sufficient to recover CRC cell growth upon arginine deprivation regardless of the basal level of ASS1 protein expression in the cancer cells and its inducibility; 2) not even an excess of citrulline, not to mention its physiological concentration, can abrogate the radiosensitizing effect of arginine deprivation combined with canavanine treatment. Here, ASS1 is linked to cancer.