Tumor endothelium is not derived from bone marrow cells. (A) Overview of the experimental strategy: BM cells from tamoxifen (TAM)-treated (2 mg tamoxifen for 2 weeks [w] every 48 hours) and untreated end-SCL-Cre-ER;R26R mice (Ly5.2/CD45.2) were transplanted into lethally irradiated congenic wild-type recipients (Ly5.1/CD45.1). After reconstitution, LLC cells (6 and 16 weeks after transplantation) or B6RV2 lymphoma cells (6 weeks after transplantation) were injected subcutaneously. Tumors were harvested and analyzed for LacZ expression 2 weeks later. Group I recipients served as the negative control group, receiving untreated end-SCL-Cre-ER^T;R26R BM and not receiving any tamoxifen treatment during tumor growth. Group II recipients received tamoxifen-treated end-SCL-Cre-ER^T; R26R marrow but did not receive tamoxifen treatment during tumor growth. Group III recipients received untreated end-SCL-Cre-ER^T;R26R marrow but received tamoxifen treatment during tumor growth. (B) Group I LLC tumors (n = 3) did not show any LacZ-positive cells. Unexpectedly, we were unable to detect any LacZ-stained cells in group III LLC (n = 3; D) and group III B6RV2 (n = 3; E) tumors. In group II LLC tumors (LacZ/CD31 costain, n = 4; C), rare LacZ⁺CD31^- cells (arrow) were observed. Original magnification × 20 (B-E). (F) Analysis of engraftment in congenic (Ly5.1/CD45.1) recipients. Peripheral blood was stained with antibodies against Mac-1, Gr-1, and CD45.2. Percentages (means, n = 6) represent proportions of peripheral blood neutrophils (Mac-1⁺Gr-1⁺), which were recipient-derived (CD45.2^-, 0.3%) and donor-derived (CD45.2⁺, 99.7%), respectively. (G) The genotype of engrafted marrow was confirmed by real-time PCR analysis of DNA extracted from peripheral blood of recipients from groups I to III.