The present invention provides an composition for suppressing the expression of a KRAS gene, comprising a lipid particle containing an double-stranded nucleic acid having an antisense strand having a sequence of a base complementary to the sequence of at least 19 continuous bases of a KRAS genes mRNA of any one of sequence Nos. 1 to 3 and a cationic lipid represented by the following formula (I): wherein R1 and R2, which are the same or different, are each a linear or branched alkyl, alkenyl or alkynyl having a carbon number of from 12 to 24 L1 and L2, which are the same or different, are each -CO-O- or -O-CO- a and b, which are the same or different, are each 1 to 3 and R3 is a hydrogen atom, an alkyl having a carbon number of from 1 to 6, or an alkenyl having a carbon number of from 3 to 6, and the like.本發明提供一種抑制KRAS基因表現之組合物等,該組合物包含反義鏈具有與序列編號1~3中任一KRAS基因之mRNA之連續之至少19個鹼基序列互補之鹼基序列的作為藥物之雙鏈核酸、及含有式(I)所表示之陽離子性脂質之脂質粒子,(式中,R1及R2為相同或不同,而為碳數12~24之直鏈狀或支鏈狀之烷基、烯基或炔基,L1及L2為相同或不同,而為-CO-O-或-O-CO-,a及b為相同或不同,而為1~3,R3為氫原子、碳數1~6之烷基或碳數3~6之烯基)。
