Provided are polypeptides with the amino acid sequences shown as SEQ ID NO : 1 and SEQ ID NO : 2, pharmaceutical compositions comprising said polypeptides, genes encoding said polypeptides, expression vectors and host cells comprising said genes, as well as uses of said polypeptides for manufacturing medicaments for the treatment of cancers. Said polypeptides can be used alone to treat or prevent cancers and can enhance markedly the ability of chemotherapeutics to kill cancer cells selectively when used combined with chemotherapeutics, with little cytotoxicity to normal cells, thus effectively decrease the toxicity and side-effect of chemotherapeutics. Also provided are products of said polypeptides conjugated with agent which can enhance the accumulation of said polypeptides in cells.