Fig. 1. The first lymphoid cells with spontaneous RAG1 expression arise within embryos on day 10.5. (A) Single cell suspensions were made from whole embryos or the indicated tissues (embryo proper, yolk sac, placenta), as described in the Materials and methods. Percentages indicate the frequencies of RAG1/GFP⁺ cells among viable 7AAD^- CD45⁺ hematopoietic cells. The results are representative of three independent experiments. (B) The bar graphs depict numbers of 7AAD^- CD45⁺ cells (left) or 7AAD^- CD45⁺ RAG1/GFP⁺ cells (right) in the indicated tissues. The data represent mean values with standard deviations from four embryos and are representative of three independent experiments.