Publisher Summary This chapter discusses the nuclear transfer in mammalian embryos. Transfer of nuclei from one cell to another provides a powerful method to study the interactions of the cytoplasm of one cell with the nuclei of another. Nuclei from various differentiated states can be transferred to nondifferentiated cytoplasm and the effect on the nucleus can be monitored. Early mammalian development is characterized by an initial period of zygotic inactivity followed by a transition period during which development is directed by both maternally stored message RNA and newly synthesized RNA. One method for determining the onset of zygotically produced transcripts is to block their appearance and subsequent protein production with α-amanitin. α-amanitin blocks mRNA synthesis by binding to a subunit of polymerase II, thus, blocking chain elongation. The chapter illustrates the importance of species comparisons and the danger of extrapolating results from one species to another. Nuclear transfer for cloning has been successful in a variety of mammals, excluding the mouse. This suggests that careful between-species comparisons should be made before any overall hypothesis regarding nuclear transfer is made. The comparisons need to include an evaluation of the advanced stage of donor nuclei capable of promoting development and the length of the cell cycle of these nuclei, the components aid in promoting development, and evaluation of nonnuclear inheritance.