Although arginine can be endogenously synthesized from citrulline and aspartate via the two rate-limiting enzymes, ASS1 and ASL, some types of cancer cells are defective in this de novo synthesis pathway due to the lower expression of these two critical enzymes [45], which makes these cancer cells become arginine auxotroph and highly addictive to exogenous arginine [46]. The gene discussed is ASL; the disease is cancer.