Abrogation of Mer expression in the E2A-PBX1⁺ human B-ALL cell line 697. Cells were infected with lentiviral particles containing short hairpin RNA (shRNA) constructs targeting Mer (shMer1A, shMer1B) or GFP (shControl) as a nonsilencing control. Mer knockdown was confirmed by Western blot analysis of whole-cell lysates (A-B) and flow cytometry (C). (A) Equal amounts of total protein were analyzed to qualitatively demonstrate reduced expression of Mer (∼ 180 kDa). The blot was stripped and reprobed with anti-Actin antibody (∼ 43 kDa) to confirm similar loading of protein. (B) Densitometric analysis of semiquantitative Western blots demonstrated significant knockdown in the clonal lines. *P < .001 vs shControl, 1-way ANOVA followed by Tukey multiple comparison test. No significant differences between shControl and wild-type were observed. Three independent measurements and mean are displayed. (C) Control and Mer knockdown 697 cells were stained with phycoerythrin-conjugated (PE) anti-Mer antibody or PE secondary antibody as an isotype control (no primary antibody). Representative histograms from 3 independent experiments are shown.