ASS1 and neoplasm: In addition, in tumor cells, the decrease or loss of argininosuccinate synthase 1 (ASS1), an important metabolic enzyme in the urea cycle, induces the accumulation of aspartic acid, an important substrate in pyrimidine synthesis, which enhances the activity of carbamoyl‐phosphate synthetase (CAD), leads to an increase in pyrimidine synthesis, and finally promotes the proliferation of tumor cells.