ASS1 and neoplasm: Normal cells synthesize arginine de novo from citrulline and aspartate via ATP in the urea cycle, however this is dysregulated in arginine auxotrophic cancers––exemplified by loss of the tumor suppressor argininosuccinate synthetase 1 (ASS1)––rendering arginine essential for tumor growth (“arginine auxotrophy”) and thereby leveraging arginine deprivation as an attractive therapeutic strategy.6, 7