DN1 and DN2 thymocytes develop to macrophages with high frequency, which can contain rearranged TCRβ genes. (A) Limiting dilution assay of DN1 (□) and DN2 (⋄) thymocytes. Replicate cultures containing 2.5, 5, 10, and 20 sorted cells were plated in 96-well plates on ST-2 stromal cells in the presence of IL-7. After 3 weeks, the number of wells containing macrophage colonies was counted under an inverted light microscope. (B) gDNA isolated from DN2 thymocyte-derived macrophage colonies was assayed by PCR for TCRβ chain rearrangement. Lane 1, TCR Dβ1-Jβ1 rearrangement of gDNA isolated from total thymocytes; lane 2, TCR Dβ1-Jβ1 rearrangement observed in a single macrophage colony; lane 3, macrophage colony with TCRβ locus in germline configuration; and lane 4, molecular weight marker.