c-Kit^lo and c-Kit^hi progenitors display different differentiation potential when grown on OP9 and OP9-DL1 cells. The following subsets of Lin^–CD44⁺CD25^–DN1 cells were sorted: c-Kit^loSca-1⁺ cells from the thymus, wt LNs, and OM⁺ LNs, and c-Kit^hiSca-1⁺ cells from the thymus. These DN1 cell populations were plated on a confluent monolayer of (A) OP9 cells or (B) OP9-DL1 cells and analyzed by flow cytometry at the indicated time points. (C) Fold expansion of progenitors plated on OP9-DL1 cells was measured by dividing the number of cells harvested by the number of cells initially plated. The analysis is of 1 representative experiment out of 3.