To assess whether the presence of a sequence-specific response to dsRNA was a peculiarity of P19 cells or whether it also extended to normal murine embryonic cells,we performed similar silencing assays in mouse embryonic stem cells. Cotransfection of embryonic stem cells with noncognatedsRNAs(e.g.,GFP),again,had no dramatic effect on either the absolute values or the ratios of Renilla and firefly luciferase activity (Fig. 3).