RNA editing defines a molecular process by which a nucleotide sequence is modified in the RNA transcript and results in an amino acid change in the recoded message from that specified in the gene. We will restrict our attention to the type of RNA editing peculiar to mammals, i.e., nuclear C to U RNA editing. This category of RNA editing contrasts with RNA modifications described in plants, i.e., organellar RNA editing (reviewed in Ref 1). Mammalian RNA editing is genetically and biochemically classified into two groups, namely insertion-deletional and substitutional. Substitutional RNA editing is exclusive to mammals, again with two types reported, namely adenosine to inosine and cytosine to uracil (C to U). This review will examine mammalian C to U RNA editing of apolipoproteinB (apoB) RNA and the role of the catalytic deaminase Apobec-1. We will speculate on the functions of Apobec-1 beyond C to U RNA editing as implied from its ability to bind AU-rich RNAs and discuss evidence that dysregulation of Apobec-1 expression might be associated with carcinogenesis through aberrant RNA editing or altered RNA stability.