Inhibition of calreticulin expression by siRNA restores CEBPA protein expression. CBFB-SMMHC protein was induced in U937 cells by withdrawal of tetracycline. In addition, cells were either electroporated with siRNA designed to knock-down calreticulin or with mock siRNA as control, simultaneously with the onset of CBFB-SMMHC induction. (A) Quantitative RT-PCR of calreticulin mRNA at day 0 (▦) and 48 hours after induction of CBFB-SMMHC transfection with or without transduction with siRNA (▪). “Control” represents U937 cells after 48 hours of withdrawal of tetracycline and thus induction of CBFB-SMMHC, whereas “mock” represents U937 cells 48 hours after withdrawal of tetracycline and transduction with the Silencer Negative Control no. 2 siRNA (Ambion) to exclude unspecific effects of the transduction procedure. Mean values and SDs are shown. (B) Western blot analysis of calreticulin, CEBPA, CEBPB, and β-actin protein after withdrawal (-) of tetracycline or transduction (or both) with (+) calreticulin siRNA in the conditional U937 inv(16) cell line (left panels) and the parental U937-T cell line (right panels).