The invention provides, among other things, methods for performing RNA interference (RNAi) in stem cells (such as embryonic stem cells) and methods for using such stem cells in vivo. The invention also provides various animal models based on conditional/inducible, reversible, tissue-specific/spacial, and/or developmental stage-specific/temporal RNAi of certain target genes, which animal model may be useful for, e.g., drug target identification and/or validation.