Libraries were made of hammerhead ribozymes with randomized binding arms and/or of U6 and tRNA-driven siRNAs. Then, the libraries were introduced into cells either by transfection or by viral vectors. This procedure made it possible to readily identify the relevant genes associated with phenotype in the apoptosis, cancer metastasis, and/or cell differentiation pathways. This application of a randomized library represents a simple and yet powerful method for identification of functional genes associated with specific phenotypes in the post-genome era. Importantly, the identified functional genes originated from not only the coding region but also the noncoding region.