Abstract Adoptive cell transfer (ACT) of tumor-reactive lymphocytes has been shown to be an effective treatment for cancer patients. Studies in murine models of ACT indicated that antitumor efficacy of adoptively transferred T cells is dependent on the differentiation status of the cells, with lymphocyte differentiation inversely correlated with in vivo antitumor effectiveness. T-cell in vitro development technologies provide a new opportunity to generate naive T cells for the purpose of ACT. In this study, we genetically modified human umbilical cord blood–derived hematopoietic stem cells (HSCs) to express tumor antigen-specific T-cell receptor (TCR) genes and generated T lymphocytes by coculture with a murine cell line expressing Notch-1 ligand, Delta-like-1 (OP9-DL1). Input HSCs were differentiated into T cells as evidenced by the expression of T-cell markers, such as CD7, CD1a, CD4, CD8, and CD3, and by detection of TCR excision circles. We found that such in vitro differentiated T cells expressed the TCR and showed HLA-A2–restricted, specific recognition and killing of tumor antigen peptide–pulsed antigen-presenting cells but manifested additional natural killer cell–like killing of tumor cell lines. The genetic manipulation of HSCs has broad implications for ACT of cancer. [Cancer Res 2007;67(6):2425–9]