CD1a⁻CD3⁻CD4⁺CD8⁻thymocytes do not contain T-cell precursors. The ability of CD1a⁻CD3⁻CD4⁺CD8⁻thymocytes to give rise to T cells was tested in a human/mouse FTOC. As a comparison, the developmental capacity of CD1a⁺CD4⁺ ISP thymocytes was examined.