The present invention relates to siRNAs that are targeted to RNAs encoding two or more enzymes of a subfamily of cytochrome P450 (CYP) enzymes, along with vectors, cells, and kits comprising the siRNAs. The invention further relates to methods of decreasing expression of two or more CYP subfamily genes in a non-human animal, animals in which expression of two or more CYP subfamily genes has been decreased, and methods of using such animals to study the function of cytochrome P450 enzymes.