Thymocytes are readily detectable in nonirradiated recipients 5 and 6 days after transplantation of CLPs but not MPPs. As schematized, CLPs or MPPs from CD45.2, Thy1.2 mice were injected into unirradiated F1 recipients (CD45.1, Thy1.1xCD45.2, Thy1.2). Five or 6 days after transplantation, as indicated, thymuses were removed and donor cells, which were not detectable over background by flow cytometry, were enriched as described in “OP9-DL1 culture,” and then cultured on OP9-DL1 stroma in the presence of 5 ng/mL of IL-7 and Flt3L for 8 days. Cultures were then analyzed for donor chimerism by flow cytometry of the entire culture (CD45.2⁺CD45.1⁻Thy1.2⁺Thy1.1⁻ cells). As shown in the table, 7 of 10 CLP recipient thymuses contained donor thymocytes that were detected in this assay, whereas only 1 of 10 MPP recipients had detectable donor thymocytes. Readouts were considered positive if the number of cells in the donor gate exceeded the number obtained from the uninjected control mouse in the same experiment. Readouts from the uninjected control ranged from 0 to 36 cells, whereas positive readouts ranged from 300 to 51 000 cells. The statistical significance of the different readouts between CLPs and MPPs was calculated using the Fisher exact test.