Nuclear transfer has been used to study the differentiation process in embryogenesis, as well as a method to produce multiple identical individuals. When nuclei are transferred to activated, enucleated oocytes the nuclei swell in diameter, synthesize DNA, acquire cytoplasmic proteins and release nuclear proteins. This protein exchange is thought to result in specific genomic modifications resulting in the transferred nucleus behaving as a zygotic nucleus. The limitations of development observed with relatively differentiated nuclei are thought to result from asynchronies in the length of the cell cycle between the donor cell and the recipient cytoplasm, as well as insufficient genomic modifications. This results in incomplete DNA synthesis and incomplete reprogramming before the first cell division. These nuclear modifications are discussed with data from amphibians and mammals.