CCR7/CCR9 DKO progenitors have an absolute defect in thymic settling in the competitive scenario. (A) Mixed BM chimeras were generated using CD45.2 WT BM and CD45.1 WT BM mixtures (top row) or CD45.2 CCR7/CCR9 DKO BM and CD45.1 WT BM mixtures (bottom row). Chimeric mice were analyzed by flow cytometry 10 weeks after BM transfer for donor chimerism of the indicated populations using antibodies to CD45.1 and CD45.2. Representative FACS plots of BM LSKs (left column), thymic ETPs (middle column), and thymic DPs (right column) are shown. (B) Shown is the mean CD45.2 donor chimerism ± SEM for each indicated population from control WT chimeras (top panel) and CCR7/CCR9 DKO chimeras (bottom panel) described in panel A. **P < .01 for the CD45.2 donor chimerism of the indicated population compared with HSC CD45.2 donor chimerism. (C) For adoptive transfer experiments, BM from either WT or CCR7/CCR9 DKO (both CD45.2) was administered intravenously (left 2 panels) or intrathymically (right 2 panels) into unirradiated WT recipient mice (CD45.1). After 2 weeks (intrathymic transfer) or 3 weeks (intravenous transfer), donor-derived thymocytes were quantified by flow cytometry. Results show the mean ± SEM for 5 recipients in each group. **P < .01 compared with the WT control.