Production of genetically identical sets of mammals: cloning?

In the past few years, there has been phenomenal progress in producing genetically identical mammalian multiplets by asexual means. The simplest method is to divide embryos into two, three, or four parts, and transfer the parts to surrogate mothers for gestation to term. Another approach is to inject nuclei from totipotent embryonic cells into fertilized one-cell embryos and to remove both male and female pronuclei. When these embryos reach the blastocyst stage, nuclei from their totipotent cells can be removed and injected into one-cell embryos, thus amplifying the process ad infinitum. Subsets of these embryos can be stored in liquid nitrogen to make genetic material for additional copies available indefinitely. Some of these techniques are fairly efficacious, others are not. No reliable techniques are available for making genetic copies of nonhomozygous adult mammals (or other vertebrates) unless samples of embryonic cells were cryopreserved when the individuals were embryos. However, injection of nuclei of spermatogonia into enucleated one-cell embryos is a promising strategy. The genetic totipotency of these nuclei is obvious since they become gamete nuclei. Moreover, the birth of mice from the diploidized genetic material of a single X-bearing spermatozoan after removal of the female pronucleus from a one-cell fertilized embryo suggests that we should eventually be able to make genetic copies of adult male mammals. This approach would not work for adult females because their oogonia have all become oocytes, and the DNA configuration in oocytes is not identical to somatic cells due to meiotic crossing over.