Single cell suspensions were created from d7EB cells via brief trypsinization. The cells were then fluorescently labeled via incubation with 10 mol/L of carboxy-fluorescein diacetate (Green Tracker, C2925; Invitrogen Corp.) in serum-free medium for 30 minutes at 37°C. Labeling was confirmed at fluorescence microscopy, and cells were kept on ice until use. Carboxy-fluorescein diacetate-labeled green fluorescent cells, 5 ϫ 10 5 , were injected i.v. through the facial vein. For cell tracking studies, mice were sacrificed at 6, 18, and 24 hours after injection, and 5-m snap-frozen lung sections were cut. Tissues were counterstained with DAPI (4=,6-diamino-2phenylindole) and examined for native green fluorescence at confocal microscopy (Zeiss LSM 510 META confocal microscope; Carl Zeiss AG, Oberkochen, Germany). Sections were graded using a semiquantitative scale established by counting the number of fluorescent cells per 100 nuclei per power field examined at 40ϫ magnification (0 ϭ no florescence, 1 ϭ Յ1 fluorescent cell, 2 ϭ 1 to 5 fluorescent cells, 3 ϭ Ͼ5 fluorescent cells). Ten power fields per slide were examined by two independent reviewers blinded to the animal group.