The function of CDADC1 (cytidine and dCMP deaminase domain containing 1, Ensembl gene identifier ENSG00000102543) is as follows. Catalyzes the deamination of dCTP to dUTP, exhibiting a strong substrate preference for dCTP over CTP (PubMed:40324085, PubMed:40504152). No deaminase activity toward 5-methyl-dCTP, cytidine, or deoxycytidine (PubMed:40324085, PubMed:40504152). May play an important role in testicular development and spermatogenesis (PubMed:16955368). In addition to its role on endogenous nucleotides, it deaminates the active triphosphate forms of the cytidine analog drugs, gemcitabine (dFdCTP) and decitabine (5-aza-dCTP), rendering them susceptible to inactivation by deoxyuridine triphosphatase (DUT). This process limits drug-induced cytotoxicity and contributes to cellular resistance to gemcitabine and decitabine (PubMed:40504152). {ECO:0000269|PubMed:16955368, ECO:0000269|PubMed:40324085, ECO:0000269|PubMed:40504152}.