Hoxa-9^-/- primitive cells have a proliferative defect in vitro and delayed emergence of committed progenitors and mature myeloid cells, defects that are corrected by re-expressing Hoxa-9. Lin^-Sca-1⁺ marrow cells were plated in bulk culture with the same combination of cytokines as Figure 4 and grown for up to 11 days. (A) Total cell numbers in the cultures over time. (B) The number of myeloid progenitors as assayed in secondary clonogenic assays in semisolid medium. (C) FACS analysis of cultured cells for Mac-1 and Gr-1 expression after 8 days in culture.