At present, genetically modified rats have not been generated from ES cells because stable ES cells and a suitable injection method are not available. To monitor the pluripotency of rat ES cells, we generated Oct4-Venus transgenic (Tg) rats via a conventional method, in which Venus is expressed by the Oct4 promoter/enhancer. This monitoring system enabled us to define a significant condition of culture to establish authentic rat ES cells based on a combination of 20% FBS and cell signaling inhibitors for Rhoassociated kinase, mitogen-activated protein kinase, TGF-β, and glycogen synthase kinase-3. The rat ES cells expressed ES cell markers such as Oct4, Nanog, Sox2, and Rex1 and retained a normal karyotype. Embryoid bodies and teratomas were also produced from the rat ES cells. All six ES cell lines derived from three different rat strains successfully achieved germline transmission, which strongly depended on the presence of the inhibitors during the injection process. Most importantly, high-quality Tg rats possessing a correct transgene expression pattern were successfully generated via the selection of gene-manipulated ES cell clones through germline transmission. Our rat ES cells should be sufficiently able to receive gene targeting as well as Tg manipulation, thus providing valuable animal models for the study of human diseases.genetic engineering | rat | embryonic stem cells T he laboratory rat was the earliest mammalian species domesticated for scientific research and has been used as an animal model in physiology, toxicology, nutrition, behavior, immunology, and neoplasia for over 150 y (1). Despite this history, rats lag far behind mice in functional genetic studies and the generation of knockout animal models reflecting human diseases because of the absence of germline-competent rat ES cells, which are vital in a reverse genetics approach (2, 3). Recently, gene-targeting rats were created by the zinc finger nuclease strategy (4). However, the system is not available for most researchers because a special technique is required to make algorithm-based sequence-specific DNA nucleases. Thus, establishment of rat ES cells has been desired to produce gene-targeting rats, such as mutant mice, routinely.Although we established rat ES cell lines with chimeric contribution, none could complete germline transmission (5). Soon after our report, other groups succeeded in establishing rat ES cells with germline transmission by using 2i, mitogen-activated protein kinase (MEK) inhibitor PD0325901, and glycogen synthase kinase-3 (GSK3) inhibitor CHIR99021 (6, 7). The 2i is widely used in the establishment of ES cells or induced pluripotent stem (iPS) cells in mice (8, 9), rats (6, 7, 10), and humans (10). Thus, the inhibition of MEK and GSK3 has been thought to maintain a ground state of pluripotency in various species. Rat iPS cells with chimeric contribution were established by using an inhibitor of type 1 TGF-β receptor Alk5 (A-83-01) with the 2i, although germline transmission was not accomplished (10). Furthermore...