shRNA (short hairpin RNA) is an artificial RNA with tight hairpin turns synthesized in the cell by DNA vector mediated by RNA pol III or a modified pol II promoter. Different form siRNA, shRNA is synthesized in the nucleus, further processed and transported to the cytoplasm, and incorporated into RISC, and then silences small molecules expressed by target genes through RNA interference (RNAi) (Figure 1). The shRNA expressed in a cell is usually delivered by a plasmid or a viral or a bacterial vector. And now, shRNA is a favorable medium for RNAi because of its relatively low degradation and conversion rate.