HOXA9−/− mice have a blunted response to G-CSF. Cohorts of (▴) 3 HOXA9−/− mice and (○) 3 HOXA9+/+ siblings were treated with twice-daily subcutaneous human G-CSF (Amgen, Thousands Oaks, CA) for 96 hours and were then bled 3 hours after the last injection. White blood cell counts were determined on a Coulter counter, and 200-cell manual differential counts of granulocytes and lymphocytes were performed on Wright-stained blood smears.