The present invention provides compositions and methods for the delivery of interfering RNAs such as siRNAs that silence APOB expression in cells such as liver cells. In particular, the nucleic acid-lipid particles provide efficient encapsulation of nucleic acids and efficient delivery of the encapsulated nucleic acid to cells such as liver cells in vivo. The compositions of the present invention are highly potent, thereby allowing effective knock-down of APOB at relatively low doses. In addition, the compositions and methods of the present invention are less toxic and provide a greater therapeutic index compared to compositions and methods previously known in the art.