TfR^-/- chimeras fail to produce mature B or T cells in the circulation. Flow cytometry was performed on blood from TfR chimeras generated on a RAG2^-/- background. Nucleated blood cells were stained with Ly9.1 (an antibody recognizing cells of 129 strain origin), B220 (pan B-cell marker), and Thy1.2 (pan T-cell marker). Cells (10 000-50 000) were counted for each sample. The plots shown for the TfR^-/- chimera are representative of all 6 chimeras obtained. Percentages of cells in each quadrant are indicated. Fewer total cells were counted for the 129 control, therefore fewer cells appear on the dot plot.