Meis1 and Meis2 maintain the proliferation of CD41⁺ hematopoietic progenitors from ES cells cultured on OP9 with hematopoietic cytokines. (A) A2lox.Meis1 (left panel) or A2lox.Meis2 (right panel) ES cells were differentiated as in Figure 3A and analyzed for total cell numbers and CD41 expression on the indicated days. Absolute CD41⁺ cell numbers were determined from the product of total cell counts, and the percentage of CD41⁺ expressing cells was determined by FACS. (B) Cells differentiated as in panel A were pulsed with BrdU for 3 hours on day 8 or day 11 and analyzed by FACS for CD41 expression and BrdU incorporation. Numbers indicate the percentage of BrdU⁺ CD41⁺ cells. (C) Cells differentiated as in panel A were analyzed by FACS for staining with annexin V and 7-AAD to label apoptotic cells and with anti-CD41 antibody to label hematopoietic progenitors. Numbers indicate the percentage of CD41⁺ cells within each quadrant.