Synthetic small interfering RNA (siRNA) duplexes are widely used to transiently and sequence-specifically disrupt gene expression in mammalian cultured cells. The efficiency and specificity of mRNA cleavage is partly affected by the presence of the nontargeting "passenger" or "sense" siRNA strand, which is required for presentation of the target-complementary or guide siRNA strand to the double-strand-specific RNA silencing protein machinery. We show that siRNA duplexes can be designed that are solely composed of two fully target-complementary guide strands that are sufficiently complementary to each other to form stable duplexes with characteristic 3' overhanging ends. The general feasibility of this approach is documented by transient knockdown of lamin A/C and emerin in HeLa cells. The silencing efficiencies of guide-only siRNA duplexes are comparable to prototypical fully paired passenger/guide duplex siRNAs, even though guide-only siRNA duplexes may contain a significant number of nonWatson-Crick and G/U wobble base pairs. Such siRNA duplexes may offer advantages regarding production costs and specificity of gene silencing.