English: Four techniques for restoring developmental potential to a somatic nucleus have been described in the literature. In nuclear transfer, the genetic material of an oocyte or zygote is replaced with that of a differentiated cell such as a fibroblast. Following development to the blastocyst stage, pluripotent ntES cells can be derived as from fertilized embryos. In cellular fusion, hybridization between ES cells and somatic cells yields tetraploid ES cell lines. In direct reprogramming, the retroviral-mediated introduction of a small number of transcription factors is sufficient to confer a pluripotent phenotype. Finally, explantation of testes tissue from neonatal and adult mice into appropriate culture conditions has been shown to result in the production of multipotent adult spermatagonial (MAS) cells.