Cytidine and deoxycytidylate deaminase zinc-binding region. The family contains cytidine deaminases, nucleoside deaminases, deoxycytidylate deaminases and riboflavin deaminases. Also included are the apoBec family of mRNA editing enzymes. All members are Zn dependent. The zinc ion in the active site plays a central role in the proposed catalytic mechanism, activating a water molecule to form a hydroxide ion that performs a nucleophilic attack on the substrate.