Probable C to U editing enzyme whose physiological substrate is not yet known. Does not display detectable apoB mRNA editing. Has a low intrinsic cytidine deaminase activity. May play a role in the epigenetic regulation of gene expression through the process of active DNA demethylation. cytidine(6666) in apoB mRNA + H(+) + H2O = NH4(+) + uridine(6666) in apoB mRNA Binds 1 Zn(2+) ion per subunit. Homotetramer. Expressed exclusively in heart and skeletal muscle. Belongs to the cytidine and deoxycytidylate deaminase family.