CDADC1 (cytidine and dCMP deaminase domain containing 1) is a vertebrate-specific enzyme that catalyzes the deamination of cytidine, deoxycytidine, and their triphosphate forms into uridine, deoxyuridine, and dUTP, respectively 1. Structurally, CDADC1 contains two deaminase domains with zinc-binding capability and forms trimeric and hexameric assemblies in solution 2. The enzyme displays a 2-fold preference for dCTP over CTP as substrates 2. Unlike bacterial homologs, CDADC1 is dispensable for mammalian cell proliferation and normal development, as Cdadc1-deficient mice show normal growth and lifespan 3. However, CDADC1 has emerged as a critical modulator of chemotherapy sensitivity through its unexpected role in metabolizing nucleotide analogs gemcitabine and decitabine 3. By deaminating the active triphosphate forms of these drugs, CDADC1 promotes their inactivation, thereby reducing therapeutic efficacy and conferring intrinsic chemoresistance in cancer cells 3. Notably, Cdadc1-deficient mice exhibit hypersensitivity to gemcitabine treatment. Gene expression studies suggest CDADC1 may contribute to testicular development and spermatogenesis 4, with developmental stage-specific expression patterns in testis, though this function requires further characterization.