Nuclear transfer technology in mammalian cloning

The past several years have witnessed remarkable progress in mammalian cloning using nuclear transfer (NT). Until 1997 and the announcement of the successful cloning of sheep from adult mammary gland or fetal fibroblast cells (1), our working assumption was that cloning by NT could only be accomplished with relatively undifferentiated embryonic cells. Indeed, live offspring were first produced by NT over 15 years ago from totipotent, embryonic blastomeres derived from early cleavage-stage embryos. However, once begun, the progression to somatic cell cloning or NT employing differentiated cells as the source of donor nuclei was meteoric, initially involving differentiated embryonic cell cultures in sheep in 1996 (2) and quickly thereafter, fetal or adult somatic cells in sheep, cow, mouse, goat, and pig. Several recent reviews provide a background for and discussion of these successes (3-6). Here we will focus on the potential uses of reproductive cloning along with recent activities in the field and a discussion concerning current interests in human reproductive and therapeutic cloning.