RNA interference (RNAi) is a sequence-specific silencing phenomenon, which is induced by double-stranded RNA (dsRNA) and mediated through an evolutionarily conserved mechanism from plants to mammals. In mammalian cells, it has recently been reported that 21- or 22-nucleotide (nt) RNAs with 2-nt 3' overhangs (siRNA) induce RNAi without induction of the dsRNA-dependent inhibition of protein synthesis, known as the host defense system against viral infections. Moreover, we and other have developed siRNA expression systems utilizing a pol III promoter. Here we report a comparative analysis among various siRNA expression vectors and also demonstrate a regulatable RNAi in cells by using a tetracycline-controlled U6 promoter.