Small interfering RNA (siRNA) is normally designed to silence preselected known genes. Such selections are inevitably prone to bias as a result of limited knowledge about the biological process, transcript identity, and functions. A library that contains all permutations of siRNA could avoid such problems. In this paper, it is shown that 5 x 10(7) siRNA-encoding plasmids can be constructed in a single tube by using vectors with two mutated RNA polymerase III promoters arranged in a convergent manner. Such a library was used to carry out genomewide screening of functional genes in a phenotype-driven manner. Multiple siRNAs that induce a significant increase of cell proliferation speed were identified.