Catalyzes the hydrolytic deamination of cytosine to uracil or 5-methylcytosine to thymine. Is involved in the pyrimidine salvage pathway, which allows the cell to utilize cytosine for pyrimidine nucleotide synthesis. cytosine + H(+) + H2O = NH4(+) + uracil Pyrimidine metabolism; UMP biosynthesis via salvage pathway; uracil from cytosine: step 1/1. Homodimer. Belongs to the cytidine and deoxycytidylate deaminase family.