Negative feedback of Pbx1 by Meis1. (A) Western blot showing Meis1, Pbx1, and actin (loading control) levels in control and Meis1-deleted lineage-negative BM. (B) Bar graph depicting Meis1 and Pbx1 mRNA levels in control and Meis1-deleted, lineage-negative BM. mRNA levels were measured by real-time quantitative RT-PCR using TaqMan primer/probes and normalized to β-actin (n = 2). (C) Lineage-negative BM cells were harvested from tamoxifen-treated Meis1-flox/CreER (Meis1-deleted) and Rosa26CreER (control) mice. Cells were transduced with lentivirus expressing 1 of 2 Pbx1-shRNAs or a control construct, and transduced cells were selected by culture in puromycin for 72 hours. Colony assay was then performed as described in the text. Bar graph depicts means ± SEM of triplicate colony counts from one representative experiment.