Effect of hGM-CSF on differentiation of TN thymocyte precursor subsets. Pro-T (CD44⁺25⁺), pre-T (CD44⁻25⁺), and post-pre-T (CD44⁻25⁻) cells were sorted from hGMR-tg mice and transferred into 2-deoxyguanosine depleted wild-type fetal lobes, as described in the Materials and Methods, and then cultured with or without 50 ng/mL of hGM-CSF. Pro-T–cell cultures were harvested on day 23, pre-T–cell cultures on day 14, and post-pre-T–cell cultures on day 9. Harvested cells were stained for expression of CD4, CD8, and αβ- or γδ-TCR. CD4 and CD8 expression is indicated by dot plot with quadrant percentages shown. Cells within each quadrant were further analyzed for TCRαβ (middle group of histograms) and TCRγδ expression (right-hand group of histograms).