HOXA9 induces Pim1 mRNA expression and binds to the Pim1 promoter region in primary hematopoietic cells. RNA isolated from U937 cells (A) and HSC-enriched Hoxa9^−/− marrow cells (B) transduced as described in Figure 1 was subjected to quantitative real-time RT-PCR analysis. Wild-type HOXA9, but not mutant HOXA9, significantly increased Pim1 mRNA levels (*P < .01). The experiments depicted in panels A and B were repeated 3 times. (C) Chromatin immunoprecipitation assays were performed on chromatin isolated from HOXA9-immortalized primary murine cell lines using either a HOXA9 antibody or IgG as a negative control. Using PCR primers to a distal segment of the Pim1 promoter, significant binding of HOXA9 to this region was demonstrated. Primers to 2 more proximal regions of the Pim1 promoter failed to show binding by HOXA9 (data not shown).