In adenovirus-derived gene therapy, one of the problems is the difficulty in specific targeting. We have recently demonstrated that monoclonal antibody (mAb) libraries screened by fiber-modified adenovirus vector (Adv-FZ33), which is capable of binding to immunoglobulin-G (IgG), provide a powerful approach for the identification of suitable target antigens for prostate cancer therapy. Hybridoma libraries from mice immunized with androgen-dependent prostate cancer cell line LNCaP were screened and mAb were selected. Through this screening, we obtained one mAb, designated LNI-29, that recognizes a glycoprotein with an apparent molecular mass of 100 kD. It was identified as neural cell adhesion molecule 2 (NCAM2). Some prostate and breast cancer cell lines highly expressed NCAM2 whereas normal prostate cell lines expressed NCAM2 at low levels. In contrast to the low efficiency of gene transduction by Adv-FZ33 with a control antibody, LNI-29-mediated Adv-FZ33 infection induces high rates of gene delivery in NCAM2-positive cancers. NCAM2-mediated therapeutic gene transduction of uracil phosphoribosyltransferase (UPRT) had a highly effective cytotoxic effect on NCAM2-positive cancer cells, whereas it had less of an effect in cases with a control antibody. In conclusion, NCAM2 should be a novel gene therapy target for the treatment of prostate and breast cancer. (Cancer Sci 2011; 102: 808–814)