[Cloning of mammals--biological aspects]

Cloning by somatic cell nucleus transfer into enucleated oocytes represents a very straightforward approach. The nucleus is simply transferred into the oocyte from which its nuclear genetic material (chromosomes) has been earlier removed. By using this approach clones of sheep, cattle, horse, rabbit, goat, rat, mule, pig, cat and interspecies-specific clone mouflon x sheep were obtained. The success of cloning is, however, disappointingly low and only 1-3% of reconstructed embryos develop to term. Some cloned animals are abnormal and die prematurely. This clearly speaks against the use of cloning in humans. On the other hand, so called therapeutic cloning, when the nucleus is transferred into an oocyte and the developing embryo is then used for the production of embryonic stem cells is generally well accepted. This promising technique may be probably used in some as yet untreatable diseases and in certain serious injuries.