Editing of the mRNA coding for apolipoprotein B involves a cytidine to uridine transition at nucleotide 6666 and enables translation of two protein variants. The development of in vitro editing systems has led to the identification of three sequence requirements in this process. The mechanism for C-->U editing requires specific sequences for editing site recognition, positioning of the catalytic activity and enhancement of catalytic efficiency. The dependence of in vitro editing on extract proteins has focused future research in this field on the identification of factors involved in apoB mRNA editing and the role of these factors in the assembly of ribonucleoprotein editosomes.