Meis1 and Meis2 inhibit erythroid progenitor differentiation but increase CD41⁺ hematopoietic progenitor differentiation from ES cells cultured on OP9 with hematopoietic cytokines. (A) A2lox.Meis1 or A2lox.Meis2 ES cells were differentiated as EBs for 6 days before plating on OP9-GFP cell monolayers and cytokines, and treated with (+) or without (−) doxycycline (dox) every 2 days until day 12. On days 7 and 8, cells were analyzed by FACS for CD71 and CD41 expression. Data shown are for cells gated for negative expression of GFP to exclude OP9 cells from the analysis. Numbers indicate the percentage of cells within each quadrant. (B) Cells were treated as in panel A and analyzed on days 9 and 12 for expression of CD42d and CD41. (C) Cells were treated as in panel A and analyzed on day 12 for expression of c-kit, AA4.1, CD45, Mac1, and Gr1.