The high affinity and specificity of peptides P51 and P25 to HER2-positive cells were confirmed in vitro by flow cytometry analysis on the HER2-positive human breast cancer cell line SKBR3 and the HER2-negative human embryonic kidney cell line 293A treated with fluorescein isothiocyanate (FITC) labeled peptides P51 and P25. Here, ERBB2 is linked to breast cancer.