Cytosine deaminase belongs to the cytidine and deoxycytidylate deaminase family. This enzyme converts cytosine to uracil or 5-methylcytosine to thymine by deaminating carbon number 4. It is present in prokaryotes and fungi but not in multicellular eukaryotes and is an important member of the pyrimidine salvage pathway. The enzyme is of widespread interest both for antimicrobial drug design and for gene therapy applications against tumors.