Pre–β rearrangement proliferation of T-cell progenitors from FL, FB, and FT. (A) IL-7R⁺ FL cells, IL-7R⁺ FB cells, and FcR^– FT cells from 12-dpc Rag2^–/– fetuses as well as CD44⁺CD25^– FT cells and CD44⁺CD25⁺ FT cells from 14-dpc Rag2^–/– fetuses were individually cultured with a dGuo-treated FT lobe under HOS conditions for 10 days. Numbers of CD25⁺ cells generated in each of the 5 to 10 clones are shown. (B) IL-7R⁺ FB cells (12 dpc) and CD44⁺CD25⁺ FT cells (14 dpc) from normal strain of mice (B6) were individually cultured under HOS conditions for 12 days. Genomic DNA was prepared from T cells generated in each well. Cells from a dGuo-treated lobe that was not seeded with a progenitor were used as a negative control. Each sample (equivalent to 750 cells) was PCR amplified using primers for Dβ1-Jβ1.6, Dβ2-Jβ2.6, or Dβ1-Jβ2.6, the locations of which are schematically shown under the figure.