Advance Publication by J-STAGE Journal of Reproduction and Development

27 Production of sires with high breeding potential is indispensable for prompt and reliable 28 breeding using their semen in the cattle industry. Currently, in Japan, we aim to further the 29 production of Japanese black sires via a new breeding system that uses genetically homologous 30 monozygotic twins so that better growth performance and carcass traits can be translated to the 31 increased production of beef with higher economic value. Several studies have reported that 32 monozygotic twins are produced by embryo bisection. On the other hand, with the evolution and 33 stabilization of in vitro fertilization technology, it has become possible to produce multiple 34 monozygotic twin calves from blastomeres separated from a cleavage-stage embryo. This 35 review attempts to clarify breeding practices through revalidation of the factors that affect the 36 production efficiency of monozygotic twin calves by embryo bisection. Furthermore, the 37 establishment of a system for monozygotic twin embryo production via the simplified technique 38 of blastomere separation is reviewed while showing data from our previously performed 39 studies. 40 41