From in vitro phage display screening, we identified two clones (PC5-2 and PC5-4) with a consensus motif of W-T/Y-Y (Table 1) . Like PC5-2, the tumor-homing ability of PC5-4 was also competitively inhibited by SP5-2 ( Fig. S4 and Text S2), suggesting that these two phages may bind through this motif to the same target molecule on the plasma membrane of lung cancer cells. We proposed that these three amino acid residues might play a crucial role in homing to tumor tissues. To test this hypothesis, we changed these three amino acid residues in SP5-2 (TDSILR-SYDWTY) to GGG in a mutant peptide, MP5-2 (TDSILR-SYDGGG). Although the tumor-homing ability of PC5-2 had been markedly inhibited by the peptide SP5-2, this competitive inhibition was lost in MP5-2, which contains the GGG residue instead of the WTY residues in SP5-2 (Fig. 2B) . These data indicate that the WTY residues were important for the binding ability of SP5-2 to NSCLC cells.