Defective thymus reconstitution of CCR7^−/−CCR9^−/− lin⁻ BM cells. (A) Lin⁻ bone marrow (BM) competitor cells (CD45.1/CD45.2) were mixed with an equal amount of lin⁻ BM cells (CD45.2) from wt or various mutant mice. Mixtures of competitor and test cells were transferred intravenously into B6 CD45.1 mice of approximately 35 days of age, and frequencies of test and competitor-derived thymocytes were assessed 21 days after transfer by staining for CD45.1 and CD45.2. (B) Statistical analysis of panel A with 5 or 6 mice per group from 2 individual experiments. Frequencies were normalized to wt versus wt controls.