Since DNMT1-depletion by decitabine and 5-azacytidine in vitro is well-documented to be impacted by expression levels of the pyrimidine metabolism enzymes DCK, UCK2, CDA, and CAD (Fig. 1a), we measured expression of these enzymes, by quantitative polymerase chain reaction (QRT-PCR), in MDS patients’ bone marrows pretreatment and at relapse on-therapy with decitabine (n = 13, median treatment-duration 175 days, range 97–922) or 5-azacytidine (n = 14, median treatment-duration 433 days, range 61–1155) (Fig. 1c). The gene discussed is CAD; the disease is myelodysplastic syndrome.