Several key components of the RNA interference (RNAi) pathway were identified in genetic screens performed in nonmammalian model organisms. To identify components of the mammalian RNAi pathway, we developed a recessive genetic screen in mouse embryonic stem (ES) cells. Recessive genetic screens are feasible in ES cells that are Bloom-syndrome protein (Blm-) deficient. Therefore, we constructed a reporter cell line in Blm-deficient ES cells to isolate RNAi mutants through a simple drug-selection scheme. This chapter describes how we used retroviral gene traps to mutagenize the reporter cell line and select for RNAi mutants. Putative RNAi mutants were confirmed using a separate functional assay. The location of the gene trap was then identified using molecular techniques such as Splinkerette PCR. Our screening strategy successfully isolated several mutant clones of Argonaute2, a vital component of the RNAi pathway.