Flow cytometric analysis of CD4 and CD8 expression on differentiating human cord blood HSCs following long-term coculture with OP9-control cells or OP9-DL1 cells. CD34⁺CD38^– cells sorted from human cord blood were cocultured with OP9-control (top 2 rows of histograms) or OP9-DL1 (bottom 2 rows of histograms) stromal cells for 52 days. The generation of DP T cells was evaluated approximately every 4 days of coculture through the co-expression of CD4 and CD8.