The psiCHECK-1 and psiCHECK-2 Vectors are designed to provide a quantitative and rapid approach for initial optimization of RNA interference (RNAi). The vectors enable monitoring of changes in expression of a target gene fused to a reporter gene. In both vectors Renilla luciferase is used as the primary reporter gene, and the gene of interest is cloned into a multiple cloning region located downstream of the Renilla translational stop codon. Initiation of the RNAi process by synthetic siRNAs or in vivo-expressed shRNAs toward a gene of interest results in cleavage and subsequent degradation of the fusion mRNA. Measuring decreases in Renilla activity provides a convenient way of monitoring the RNAi effect. In comparison with other fusion approaches (e.g, GFP or flag-tags), the Renilla luciferase approach offers more convenient and rapid quantitation with higher sensitivity. The psiCHECK-1 Vector is recommended for use in monitoring RNAi effects in live cells. The changes in Renilla luciferase activity are measured with the EnduRen Live Cell Substrate (Cat.# E6481), which allows continuous monitoring of intracellular Renilla luminescence. The psiCHECK-2 Vector contains a second reporter gene, firefly luciferase, and is designed for endpoint lytic assays. Introduction of firefly luciferase in the psiCHECK-2 Vector allows normalization of Renilla luciferase expression, achieving robust and reproducible results.