WT thymic precursors do not sustain long-term thymopoiesis in ZAP-70–deficient hosts. (A) ZAP-70^−/− mice were intrathymically transplanted with DN thymocytes or lineage-negative (lin⁻) BM progenitor cells (2 × 10⁵) isolated from CD45.1⁺ WT mice. Eight weeks later, donor engraftment was assessed by monitoring the percentages of cells expressing the CD45.1 allele. A thymus from a CD45.1⁺ WT mouse is shown as a control. (B) The phenotype of donor cells was assessed by CD4 and CD8 staining of gated CD45.1⁺ thymocytes. A representative dot plot from a control CD45.1⁺ WT mouse is shown. The percentage of cells in each gate is indicated.