One year ago, the first KO rats were reported by Geurts et al. (Science, 2009, 325:433) , using the Zn-finger nucleases technology. Half a year earlier, several laboratories had reported the pionner isolation of true rat embryonic stems (ES) cells, including a manuscript published by Li et al. (Cell, 2008, 135: 1299-1310), from Qi-Long Ying’s lab. Last week, this laboratory reported the generation of the first knockout rats obtained by homologous recombination in rat ES cells [Tong et al. (Nature, 2010, August 11)], thereby illustrating the power of those rat ES cells for the inactivation of a most popular locus, the P53 gene. This report demonstrates that the rat genome is amenable to the same type of genetic modifications as the mouse genome, using equivalent procedures.