The reconstruction of mammalian embryos by transfer of a blastomere nucleus to an enucleated oocyte or zygote allows for the production of genetically identical individuals. This has advantages for research (that is, as biological controls) and commercial applications (that is, multiplication of genetically valuable livestock). However, the number of offspring that can be produced from a single embryo is limited both by the number of blastomeres (embryos at the 32-64-cell stage are the most widely used in farm animal species) and the limited efficiency of the nuclear transfer procedure. The ability to produce live offspring by nuclear transfer from cells that can be propagated and maintained in culture offers many advantages, including the production of many identical offspring over an extended period (since cultured cells can be frozen and stored indefinitely) and the ability to modify genetically or to select populations of cells of specific genotypes or phenotypes before embryo reconstruction. This objective has been achieved with the production of lambs using nuclei from cultured cells established from embryonic, fetal and adult material. In addition, lambs transgenic for human factor IX have been produced from fetal fibroblasts transfected and selected in culture.