Catalytic component of the apolipoprotein B mRNA editing enzyme complex which is responsible for the postranscriptional editing of a CAA codon for Gln to a UAA codon for stop in the APOB mRNA. May also play a role in the epigenetic regulation of gene expression by participating in DNA demethylation. cytidine(6666) in apoB mRNA + H(+) + H2O = NH4(+) + uridine(6666) in apoB mRNA Binds 1 Zn(2+) ion per subunit. Homodimer. Part of the apolipoprotein B mRNA editing complex with APC. Interacts with HNRPAB and SYNCRIP. Expressed in the spleen. Expressed at lower level in the kidney, testis, lung, brain and liver. Belongs to the cytidine and deoxycytidylate deaminase family.