Short interfering RNAs (siRNAs) and short hairpin RNAs (shRNAs) usually used for RNA interference (RNAi) are double-stranded RNAs (dsRNAs) of 21 base pairs. However, siRNAs and shRNAs of longer stem length have been reported to show more potent gene silencing. Here, we report a new technique to enzymatically construct shRNA libraries containing clones from firefly luciferase cDNA and Jurkat cDNA. The technique allowed the efficacious generation of shRNAs of arbitrary stem length as desired, providing-the clones which potently silenced the specified gene expression and presenting a high efficiency rate of gene silencing. Our results indicate that the technique permits the rapid, efficient, and low-cost preparation of genomewide shRNA expression libraries not only for humans and mice but also for sorts of biological species and that the relevant libraries are applicable for the search of genes related to phenotype changes and of new targets for drug discovery. (c) 2006 Elsevier Inc. All rights reserved.