In vitro colony assays. Sorted CD34⁺CD1a^– and CD34⁺CD1a⁺ thymocytes and CD34⁺ UCB cells were plated in semisolid cultures containing appropriate cytokines to generate myeloid (CFU-GM) or erythroid (BFU-E) colonies. (A) Typical myeloid (left) and erythroid (right) colonies generated from CD34⁺CD1a^– thymocytes. (B) Overview of BFU-E dishes in which 10⁵ CD34⁺CD1a^– or CD34⁺CD1a⁺ thymocytes or 10³ CD34⁺ UCB cells were plated. No colonies were detected in dishes with CD34⁺CD1a⁺ thymocytes. (C) RQ-PCR analysis of immature TCRD gene rearrangements (Dδ2-Dδ3) in original CD34⁺CD1a^– thymocytes, myeloid and erythroid colonies generated from CD34⁺CD1a^– thymocytes, myeloid colonies generated from CD34⁺ UCB cells, and CD34⁺ cells from children's BM sorted into either CD13/33^– (mostly B-cell progenitors) and CD13/33⁺ (myeloid progenitors) fractions. Percentages of rearranged alleles in each sample are shown.