Typically, peptides used successfully as molecular imaging agents require KD or IC50 values below 20 nM.18–22 Given that the reported binding affinities of both peptides were much higher than this threshold, our goal was to first confirm their GPC3 binding in vitro at concentrations near their reported KD values and then alter the peptide sequences to develop higher-affinity, peptide-based imaging probes for HCC and other GPC3-expressing cancers. Here, GPC3 is linked to hepatocellular carcinoma.