ASS1 and glioblastoma: We previously demonstrated that 30% of de novo GBM patients are auxotrophic for the amino acid arginine as a result of transcriptional silencing of ASS1 and/or ASL which encode the two key enzymes in arginine biosynthesis, argininosuccinate synthetase (ASS1) and argininosuccinate lysase (ASL) respectively2,3.