This invention provides for a compound of formula (I): or a pharmaceutically acceptable salt thereof, wherein R1-R4, L and X are defined herein. The compounds of formula (I) and pharmaceutically acceptable salts thereof are cationic lipids useful in the delivery of biologically active agents to cells and tissues.本發明提供式(I)化合物:或其醫藥上可接受之鹽,其中R1至R4、L及X係如本文所定義。式(I)之該等化合物及其醫藥上可接受之鹽係可用於將生物活性劑遞送至細胞及組織之陽離子脂質。