Expansion potential of cord blood–derived HSCs following long-term coculture on OP9 cells. 1 × 10⁴ CD34⁺CD38^– cells sorted from human cord blood were seeded onto OP9-control or OP9-DL1 stromal cells and cultured for 7 weeks. Total cellularity from OP9 control (•) or OP9-DL1 (○) cocultures was determined during the course of coculture at the indicated time points. The data are presented as the mean ± standard deviation of 3 independent experiments.