Recently, we showed that ρ0 cancer cells devoid of mitochondrial DNA (mtDNA) recover tumour formation ability only after acquisition of host mtDNA. In order to restore respiration, mtDNA moves between cells in whole mitochondria. While it is now clear that recovery respiration is essential for tumour formation, its functional link to the process is unclear. It includes gradual increase in mtDNA level of homoplasmic host polymorphism, followed by binding of mtDNA-processing enzymes to its regulatory domain, replication and transcription of mtDNA, increased expression of components of respiratory complexes, resulting in full restoration of respiration. We found that pyrimidine biosynthesis, supported by the respiration-linked enzyme dihydroorotate dehydrogenase (DHODH), is critically required to overcome cell cycle arrest. We further confirmed that efficient de novo pyrimidine synthesis, necessary for tumour cell proliferation, is the key event for triggering tumour growth. Moreover, respiration recovery, which is necessary for tumour formation, is associated with efficient de novo pyrimidine synthesis. In conclusion, we propose that DHODH is a critical link between de novo pyrimidine synthesis and respiration, and that it is a promising target for broad-spectrum cancer therapy.