Cytidine deaminase (CDA) is a pyrimidine metabolism enzyme that catalyzes the deamination of cytidine and deoxycytidine to generate uridine monophosphate (UMP), functioning as a critical salvage pathway enzyme for nucleotide biosynthesis. The enzyme localizes to the cytosol and operates as a homodimer with zinc-dependent catalytic activity 1. CDA exhibits substrate specificity, with different protein variants displaying differential kinetic preferences; the Q27 variant preferentially utilizes cytidine as substrate compared to the K27 variant 1. The gene contains multiple polymorphisms that significantly influence enzyme activity levels in the population; the CDA*2B haplotype and the 79A>C (K27Q) SNP show the strongest associations with CDA activity variation 1. Clinically, CDA polymorphisms have important pharmacogenetic implications for cytidine analog-based cancer therapies. Patients receiving nucleoside analog chemotherapy benefit from CDA activity and genotype assessment to optimize therapeutic efficacy and safety 1. Additionally, CDA activity assessment may predict individual responses to anticancer treatments employing cytidine analogs. The enzyme's role in pyrimidine salvage metabolism makes it essential for maintaining adequate nucleotide pools for DNA synthesis and cell proliferation.