The other four were tumor-targeting peptides: P6 (FSH1), targeting follicle-stimulating hormone receptor30; P7 (BP9a), targeting human transferrin receptors31; P8 (RGD) targeting integrin receptors on the tumor vasculature32; and P9 (peptide 18–4), which targets keratin 1 in breast cancer cells33,34. Here, KRT1 is linked to breast cancer.