Group 1 CD1-expressing hematopoietic cells mediate the positive selection of NK1.1⁺ HJ1 T cells. CD1d^−/− or hCD1Tg/CD1d^−/− mice were reconstituted with BM cells from HJ1Tg/CD1d^−/− or HJ1Tg/hCD1Tg/CD1d^−/− mice (both donor and recipient mice are on a Rag^−/− background). Six to 7 weeks later, cells were isolated from each group, stained with mAb against CD4, CD8, CD3 and NK1.1, and analyzed by flow cytometry. (A) Dot plots depict the proportion of CD4- and CD8-expressing thymocytes in each quadrant, as well as the proportion of NK1.1-expressing T cells in the lymphocyte gate. Data are representative of 3 independent experiments with 2 mice in each group. (B) Bar graphs depict the mean ± SD for the percentages (top panel) and absolute numbers (bottom panel) of NK1.1⁺CD3⁺ T cells in the liver lymphocyte gate. Statistical significance was evaluated by comparing Groups II, III, and IV with the negative control group (Group I). **P < .01, n = 6 for each experimental group.