Apolipoprotein (apo) B100 mRNA undergoes site specific C to U editing, generating a stop-translation codon of apo B48 in the small intestine. This reaction is catalyzed in an editosome which contains APOBEC-1, a catalytic subunit. To clarify the functional significance of the apo B mRNA editing in lipoprotein metabolism, we have generated APOBEC-1 knockout mice and double knockout mice which are deficient in both APOBEC-1 and apo E. The apo B mRNA editing activity was markedly reduced and complete elimination of apo B48 from the plasma was observed in APOBEC-1(-/-) mice. Plasma triglyceride levels significantly increased in the double knockout mice (APOBEC-1(-/-);apo E(-/-)) as compared to apo E(-/-) mice. These results suggest that APOBEC-1(-/-) mice are a valuable model for experiments designed to understand a role of apo B mRNA editing.