HOXB4 specific repression of Prdm16 in HSCs in vivo. (A) Quantitative RT-PCR for expression of Prdm16 mRNA in LSK cells harvested from transplanted mice at 6 weeks and 39 weeks posttransplantation. Cells were sorted based on expression of the HOXB4-YFP vector with negative cells serving as the control population. (B) Quantitative RT-PCR for expression of Prdm16 mRNA in GFP-, HOXB4-, HOXA9-, or HOXA10-transduced CD150⁺CD48⁻LSK cells, CD150⁺CD48⁺LSK cells, and CD150⁻CD48⁺ LSK cells from 6-week transplanted mice. Downregulation of Prdm16 by HOXB4 and upregulation of Prdm16 by HOXA9 or HOXA10 were detected in HSCs only. Low or no Prdm16 expression was detected in restricted HPCs compared with wild-type LSK cells. (C) Quantitative RT-PCR for expression of Prdm16 mRNA in GFP- and HOXB4-transduced LSKs 4 days, 8 days, and 13 days posttransduction. No Prdm16 expression was detected either GFP- or HOXB4-transduced LSK cells at any time point compared with wild-type LSK cells. In all panels, the expression level of Prdm16 in wild-type LSK cells was set to a control value of 1.0 and expression of Gapdh mRNA was used as internal control. WT, wild-type.