This chapter focuses on reproducible and easily performed nuclear transfer techniques that explore the role of the major components of fertilized eggs in mouse embryonic development. The essential contribution of both the male and female genome to mammalian development was shown using the nuclear transfer method and genetic analysis. Nuclear transfer allowed the dissection of the role of the functionally unique components of zygote, and also made possible the analysis of the ability of the blastomere nuclei to support normal development after transfer into the zygote. Paternal and maternal genomes are functionally different and complementary; thus, the presence of both is required for complete development. It is likely that functional differences exist for only small numbers of specific genes, while most of the male and female genome is comparable. These somewhat unexpected results further emphasize the need for identification and characterization of genes, which regulate early mammalian development.