The present invention provides, among other things, compositions and methods of formulating nucleic acid-containing nanoparticles comprising no more than three distinct lipids components, one distinct lipid component being a sterol-based cationic lipid. In some embodiments, the present invention provides compositions and methods in which the lipid nanoparticles further comprise helper lipids and PEG-modified lipids. The resulting formulation comprises a high encapsulation percentage for nucleic acids.