Therapeutic cloning: concepts and practicalities.

The concept of using embryonic stem (ES) cells as a source of multiple cell types for use in tissue repair has existed for approximately 20 years. Recent breakthroughs in somatic nuclear transfer and human ES cell derivation have produced a flurry of new activity in this area, with the recognition that ES cell lines that are customized and genetically identical to those of the patient are a distinct possibility. This article examines the background of and prospects for these exciting new developments.