Gene targeting in ES cells: In some of the livestock species pro-nuclear injection cannot be used to modify the animals' own genes due to various reasons especially non availability of large number of fertilized eggs and translucent zona. By contrast, the use of embryonic stem (ES) cells in the mouse has enabled us successfully in developing target specific modifications. When these cells with modified DNA are inserted into the early embryo, may contribute to the germline. They undergo a homologous recombination with transferred DNA of interest resulting in to precise and targeted changes in host genome. It is because of combination of these two properties, gene targeting including knockout of some undesired genes in mouse germline is a routine procedure. Despite extensive efforts globally, stable ES cells have not been isolated from livestock and this has constrained many potential applications for germline manipulation. Therefore, it was with some excitement that researchers in the field greeted the news that cloning by nuclear transfer (NT) from cells grown in culture had been achieved as it pointed the way to the development of an alternative, cell-based, system for genetic modification.