Posttranscriptional C→U RNA editing. Following transcription, nascent pre-RNA associates with hnRNPs and splicing factors. Spliced mRNA is shown with the minimal core complex of the apoB RNA holoenzyme (apobec-1 plus ACF) in proximity to the targeted cytidine. Note that apobec-1 functions as a dimer and that the stoichiometry of binding (ACF/apobec-1) is currently unknown. The activity of the core enzyme complex is regulated through protein-protein interaction of both apobec-1 and ACF, each with a number of proteins that may together represent the larger holoenzyme complex. ACF is proposed to function in the translocation of apobec-1, as well, and is illustrated here shuttling between the nucleus and cytoplasm.