And nanoparticles and a cell-introducing agent capable of releasing a target substance in a weakly acidic pH environment. A nanoparticle comprising a peptide and a particle-forming component, wherein the particle-forming component forms a liposome or micelle, wherein in the peptide the sequence has 2 to 8 units starting with His (histidine) and ending with an acidic amino acid Each unit may be the same or different), nanoparticles.
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