Small interfering RNA (siRNA) has been widely used for suppressing gene expression in various organisms. Here, we describe efficient methods to suppress target genes (EGFP or Oct4) using siRNA in mouse and monkey ES cells, and differentiation. In mouse ES cells. FAGS analysis revealed that EGFP expression was suppressed in 97% of transfected cells at 48 h after transfection. In addition; cells expressed Hand1 and Cdx2, which are the marker genes of trophoblast lineage by the transient suppression of Oct4. In the case of monkey ES cells, highly efficient suppression was achieved in 98% of cells at 96 h post-transfection using the Sendai virus (hemagglutinating virus of Japan, HVJ) envelope as a carrier of siRNA. These efficient transfection methods using synthetic siRNA should contribute to evaluate specific gene function in ES cells and can be used to differentiate ES cells into desired cell lineages. (c) 2005 Elsevier Inc. All rights reserved.