TfR^-/- chimeras, like RAG2^-/- animals, fail to produce thymic cells that express CD4 or CD8. Flow cytometry was performed on thymocytes from controls and 7-week-old TfR^-/- chimeras generated on a RAG2^-/- background. Thymus cells were stained with Ly9.1, CD4, and CD8 antibodies. Cells (200 000-500 000) were counted in these analyses. The plot shown for TfR^-/- chimera is representative of the 6 chimeras obtained. Percentages of cells in each quadrant are indicated.