Crot (27-39) is a cationic cell-penetrating peptide, but the peptide is not very hydrophilic, so it is also called a hydrophobic cell-penetrating peptide. Since the peptide sequence contains three cysteines, the secondary structure looping phenomenon is prone to occur during the peptide synthesis and purification process, and the stability is relatively poor.The guanidine, sulfhydryl and amino groups in the Crot (27-39) sequence can play a decisive role in the process of membrane penetration.