Like plant cells, the mammalian cells can be transformed by the foreign DNA fragments. For transforming the mammalian cells, it is necessary to precipitate the DNA with calcium phosphate and mix the cells to be transformed. DNA molecule passes through cell membrane and integrates randomly with mammalian chromosome. Using this technique selective marker can be linked up with DNA fragment to be cloned and can be introduced into mammalian cells. The transformed cells are, thereafter, separated from cell line after plating them on selective medium. Many techniques are used to recover the selectable marker gene from the biochemically transformed cell line. Following these biochemical processes. Perucho et al. (1980) successfully isolated chicken thyamidine gene (tk gene) by ligating with pBR322 and transforming the tk' (deficient of thyamidine gene) mouse.