Lower levels of PU.1 expression in APL-initiating cells. (A) SP cell analysis. The SP cells were gated in the R2 box and were detected following Hoechst 33342 and verapamil co-treatment. Hoechst staining of APL mononuclear cells in the absence or presence of verapamil is shown in the upper and lower panels, respectively. The FACS profiles represent the results of the flow cytometry analysis of SP cells from APL samples. (B) Lower PU.1 levels in APL-derived SP cells as compared to UCB-derived SP cells. The PU.1 mRNA expression levels of SP cells from UCB specimens or primary APL samples were measured using real-time RT-PCR and were normalized to GAPDH expression. Each symbol represents the average value from an individual patient or UCB specimen.