The present invention provides a composition with weak cytotoxicity for introducing a nucleic acid, such as a short oligonucleotide or a gene, into a cell and expression of the gene in the cell with improved capability for introduction of the nucleic acid, and a novel compound contained in the composition. When a compound represented by a formula (I): (R1)n -R2-R3 (I)wherein (R1)n represents a polyamino acid residue consisting of a total of n amino acid residues, which are identical to or different from one another, the n residues being selected from the group consisting of an arginine residue, a lysine residue, and a histidine residue, and n being an integer of from 4 to 16 R2 represents a single bond or an amino acid residue and R3 represents a phospholipid residue with 1 or 2 identical or different unsaturated fatty acid residues having from 12 to 20 carbon atoms, and a lipid are administered or supplied to a cell together with a nucleic acid such as a gene, the nucleic acid can be efficiently introduced with weak cytotoxicity.
