ASS1 and cancer: However, in many tumors, loss ASS1 in urea cycle promotes cancer proliferation by diversion of aspartate substrate towards carbamoyl-phosphate synthase 2 (CPS2), aspartate transcarbamylase (ATC), and dihydroorotase (DHO), enzymes that catalyze the first three reactions in the pyrimidine synthesis pathway, resulting in increased tumor progression [36].