RNA interference (RNAi) has been increasingly used for reverse genetics. Both pol III and pol II promoters have been used to synthesize short hairpin RNA (shRNA) for knockdown of gene expression in mammalian cells and animals. Compared with gene knockout approaches, RNAi has the advantage of being simple, quick, and low cost. Here we describe a method that enhances this advantage where knockdown of expression of multiple genes in the same cells is required. A tetracycline-regulated pol II promoter construct allows the expression of up to three shRNA genes that have been cloned into introns of a transcript bearing green fluorescent protein (GFP) coding sequences. This method may be used to establish stable knockdown cell lines and may also prove useful for investigating gene-gene interactions in transgenic animals.