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. Also involved in CGA (Arg) to UGA (Stop) editing in the NF1 mRNA. May also 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. Homodimer. Part of the apolipoprotein B mRNA editing complex with APC. Interacts with HNRPAB and SYNCRIP. Belongs to the cytidine and deoxycytidylate deaminase family. Extended N-terminus.