The present invention provides a cationic lipid, which allow nucleic acids to be easily introduced into cells, represented by formula (I) (wherein: R1 and R2 are, the same or different, alkenyl, etc, and X1 and X2 are hydrogen atoms, or are combined together to form a single bond or alkylene, and X3 is absent or is alkyl, etc, Y is absent or anion, a and b are, the same or different, 0 to 3, and L3 is a single bond, etc, R3 is alkyl, etc, L1 and L2 are -O-, -CO-O- or -O-CO-), a composition comprising the cationic lipid and a nucleic acid, and a method for introducing a nucleic acid into a cell by using the composition comprising the cationic lipid and the nucleic acid, and the like.本發明提供一種將核酸容易地導入至細胞內之式(I)所表示之陽離子性脂質、含有該陽離子性脂質及核酸之組合物、使用含有該陽離子性脂質及核酸之組合物而將核酸導入至細胞內之方法等。(式中,R1及R2相同或不同,為烯基等,X1及X2為氫原子,或者一併形成單鍵或伸烷基,X3不存在,或為烷基等,Y不存在,或為陰離子,a及b相同或不同,為0~3,L3為單鍵等,R3為烷基等,L1及L2為-O-、-CO-O-或者-O-CO-)。
