The present invention is directed to lipid-based formulations for delivering,e.g., introducing, nucleic acid-lipid particles to a cell, and assays foroptimizing the delivery efficiency of such lipid-based formulations. Thenucleic acid-lipid particles comprise an interference RNA molecule, a cationiclipid with alkyl side chains from about 10 to about 20 carbon atoms havingmore than a single site of unsaturation, a non~cationic lipid and a conjugatedlipid that inhibits aggregation of the particle such as a polyethyleneglycol(PEG)-lipid conjugate or a polyamide (ATTA)-conjugate).
