Cytidine deaminase catalyzing the cytidine to uridine postranscriptional editing of a variety of mRNAs (PubMed:30844405). Form complexes with cofactors that confer differential editing activity and selectivity. Responsible for the postranscriptional editing of a CAA codon for Gln to a UAA codon for stop in the apolipoprotein B mRNA (PubMed:24916387). Also involved in CGA (Arg) to UGA (Stop) editing in the NF1 mRNA (PubMed:11727199). May also play a role in the epigenetic regulation of gene expression by participating in DNA demethylation (By similarity). {ECO:0000250|UniProtKB:P51908, ECO:0000269|PubMed:11727199, ECO:0000269|PubMed:24916387, ECO:0000269|PubMed:30844405}. This is the function of APOBEC1 (apolipoprotein B mRNA editing enzyme catalytic subunit 1, Ensembl gene identifier ENSG00000111701).