Lymphopoietic potential of ES cell–derived Flk1⁺ precursors. (A) A total of 10⁴ ES cells were seeded onto OP9 cell monolayers in 6-well plates. Cells were harvested from ES/OP9 cocultures on various days and analyzed by flow cytometry for Flk1 surface expression. The percent of cells expressing Flk1 is shown. Cocultures were disaggregated with trypsin on day 6. (B) A total of 2 × 10⁴ ES cell–derived Flk1⁺ cells was isolated by flow cytometric cell sorting (day 5 ES/OP9 coculture) and reseeded onto fresh OP9 cell monolayers. Evidence of B lymphopoiesis on day 14 was determined by flow cytometric analysis for the surface expression of CD45 and CD19. The total cell yield was approximately 1.5 × 10⁵ cells for the coculture shown.