FLT-1-transfected ALL cells show different in vivo phenotype. Native RS4;11 cells or those transfected with full-length FLT-1 were inoculated into sublethally irradiated NOD-SCID mice, as described in “Materials and methods.” (A) Results show the transfected cells (bottom) formed masses of leukemia cells in the epiphysis (here shown at × 400 magnification) of the long bones, 3 to 5 days after inoculation, whereas the untransfected cell line localized predominantly in the diaphysis (top, × 200 magnification). Black arrows indicate leukemia cells in the different areas of the BM of long bones; white arrows, evidence for blood leakage in the bones of inoculated recipients. Results shown are representative of 3 mice per condition. (B) Evidence for the earlier appearance of extramedullary disease in mice inoculated with FLT-1-transfected RS4;11 cells. Bottom panel shows a spleen section (× 200 magnification) of a NOD-SCID recipient 10 days after RS4;11 FLT-1T cells inoculation, clearly evidencing the engraftment of leukemia cells. The results are representative of 3 independent animals. Images were visualized through a 40 ×/0.65 PLAN objective lens, captured with a Sony Cybershot DSC-560 digital camera (Sony, Lisbon, Portugal), and processed with Adobe Photoshop software (Adobe Systems, San Jose, CA).