ASS1 and cancer: Polyamine treatment restored arginine deprivation-induced ASS1 expression to baseline (Supplementary Fig. S3E), while inhibition of polyamine synthesis enzyme ODC1 by difluoromethylornithine (DFMO) upregulated the ASS1 mRNA level (Supplementary Fig. S3F), together suggesting that cancer cells regulated arginine synthesis in accordance with the availability of polyamine.