Development and application of technology for large scale cloning of cattle.

Mammalian cloning technologies originally developed as methods of testing hypotheses about the mechanisms of cell differentiation. Embryo splitting procedures demonstrated that each of the cells in the early embryo are capable of developing into a complete new individual. Nuclear transplantation technologies have shown that loss of genetic sequences or even irreversible repression of gene function are also not mechanisms of cell differentiation. Therefore, both of these methods can be used for producing genetically identical animals. Nuclear transplantation has the advantage of being able to produce unlimited numbers of identical offspring. Highly efficient procedures have been developed for nuclear transplantation in mammals and several important characteristics of donor cells have been described. Unfortunately, the efficiency of producing cloned offspring is still low and many factors affecting the development of nuclear transfer embryos to term remain to be investigated. The tremendous potential of the technology for use in agriculture and medicine, however, will ensure that these problems are addressed and solved.