These include downregulation of deoxycytidine kinase (DCK), the enzyme that executes the rate-limiting phosphorylation step necessary for DNA incorporation of cytidine analogues, downregulation of the nucleoside transporters that mediate cellular uptake of cytidine analogues by the pyrimidine salvage pathway, and upregulation of CDA within cancer cells (reviewed in[28]). Here, CDA is linked to cancer.