To determine whether this targeting ligand had an affinity for human lung cancer surgical specimens, we tested the reactivity of PC5-2 and SP5-2 with pulmonary adenocarcinoma cells using immunohistochemistry. Both PC5-2 and biotin-labeled SP5-2 (B-SP5-2) recognized the tumor cells of NSCLC surgical specimens (Fig. 1D a and c, arrowheads) , and the control phage and biotinlabeled control peptide (B-Con-P) did not (Fig. 1D b and d) . SP5-2 (TDSILRSYDWTY) competed with PC5-2 for binding to surgical specimens of pulmonary adenocarcinoma (Fig. 1De ), but the same concentration of a mutated peptide, MP5-2 (TDSILRSYDGGG) did not (Fig. 1Df) . Seventy-five percent (27/36) of the pulmonary adenocarcinoma specimens from 36 patients expressed a target molecule that was recognized by this peptide (Table S1 ). These data indicated that SP5-2 could recognize unidentified molecules expressed on NSCLC cell lines and actual cancer cells from the surgical specimens of lung cancer.