A site-specific post-transcriptional cytidine to uridine deamination reaction is responsible for the production of apolipoprotein B48 in the mammalian small intestine. The molecular machinery responsible for apolipoprotein B RNA editing consists of apobec-1, an RNA-specific cytidine deaminase that functions in conjunction with a recently identified protein referred to as ACF/ASP. These proteins together represent the minimal editing enzyme, although other proteins may associate with the enzyme complex. Apobec-1 is a member of a supergene family of cytidine deaminases, with several homologs recently identified in the human genome. ACF/ASP is novel, and emerging information reveals interesting clues to its role in the apolipoprotein B RNA editing enzyme complex.