CAD and neoplasm: We found that even though the three proteins of the de novo pyrimidine synthesis pathway, the trifunctional CAD (carbamoyl-phosphate synthetase 2, aspartate transcarbamoylase, and dihydroorotase), DHODH and the bifunctional UMPS (uridine monophosphate synthase) are expressed on mRNA and protein level in all cell lines (Fig. 4B, C), DHODH-linked respiration is suppressed in ρ0 cells, and is restored in tumour-derived cells to an even higher level than in parental cells (Fig. 4D).