To the Editor: The recent description of highly active transcription activator-like effector nucleases (TALENs) 1 prompted us to explore their utility for in vivo genetic engineering in the laboratory rat. The rat is a valuable experimental animal because of its suitability for modeling human disease and toxicology. Zincfinger nuclease (ZFN) technology and the isolation of rat embryonic stem cells have enabled targeted modifications of the rat genome2–5. Recently, Xanthomonas-derived transcription activator-like (TAL) effector proteins have elicited much interest because of their apparently simple rules for sequence-specific DNA recognition6, 7. Several investigators have fused the FokI nuclease domain to TAL effector proteins to create TALENs1, 8–12. However, only optimal truncation of the TAL effector protein allowed high-frequency gene disruption of endogenous loci and targeted DNA integration1, 13, 14. Here we use TALENs to disrupt the rat IgM locus, creating heritable mutations that eliminate IgM function. Our results establish the use of TALEN technology for in vivo gene knockout in mammals. We designed and assembled TALENs to exon 2 of rat IgM and tested their ability to alter the IgM locus in rat S16 cells