Occupancy of niches by ETPs regulates thymus seeding. Multicongenic BM-derived progenitors were injected into DKO recipient mice, one individual tag (10⁶ of lin⁻ BM cells/recipient mouse) every 3 d for 21 d. Analysis of donor-derived splenic T cells was performed 42 d after the final injection. (A) Quantification of donor cells within all splenocytes of DKO recipient mice. (B) Quantification of donor cells within all splenocytes of WT recipient mice. Data are representative of two independent experiments, n = 7–12 recipients/genotype. Recipient mice were 5 and 6 wk old. Donor mice were 7–8 wk old.