An RGD tumor-targeting peptide, which binds to αvβ3 receptors. c(RGDfK) can be easily conjugated to fluorophore tags, chelators, or carriers. Palecek et al. converted the ε-amino group into the azide by diazo transfer allowing further functionalization by click chemistry. Mercapto groups can be introduced via acylation of c(RGDfK) with S-acetylthioacetic acid (SATA).