ETPs generate granulocytes with a developmental history of RAG-1 expression. (A) Granulocyte and cDC subsets in the thymus and spleen were examined in RAG-1/Cre × Rosa26YFP mice for the presence of YFP labeling. Shaded histograms represent the same population in YFP⁻ control mice. (B) The mean percentage of YFP⁺ cells in each population was quantified. Thymus populations are shown by black bars; splenic populations by gray bars. There is no corresponding splenic population for ETPs, which are found only in the thymus. Three mice per group were examined. Error bars represent SEM. **P < .01 for the percentage of YFP⁺ cells of the indicated population in the thymus compared with the analogous population in the spleen. (C) ETPs plus DN2 thymocytes or DN3 thymocytes were sorted from CD45.2⁺ RAG-1/Cre-Rosa26YFP mice and intrathymically injected into congenic CD45.1⁺ sublethally irradiated recipients (7000 ETP + DN2/recipient and 70 000 DN3/recipient). Donor contribution to YFP⁺ thymic Mac1⁺Gr1⁺ cells was examined 6 days after injection. (D) ETPs (10 000/recipient) and DN3 thymocytes (50 000/recipient) were sorted from CD45.2⁺ donor mice and intrathymically injected into sublethally irradiated CD45.1⁺ recipients. Mice were killed 6 or 11 days after injection and examined for donor contribution to Mac1⁺Gr1⁺ thymic granulocytes. Four mice per group were examined.