Recent progress in experimental mouse embryology has provided new approaches to the genetic manipulation of the mammalian embryo. The production of uniparental embryos enables one to compare maternal and paternal gene activity during development, to study the biological consequences of homozygosity of mutant genes, and to further elucidate the unsolved problem of X chromosome inactivation. Transplantation of nuclei from somatic cells into mouse eggs is considered the most vigorous functional test for the developmental capacity of the nuclear genome and provides a bioassay for the study of possible genomic changes during cellular differentiation. Transplantation of cloned eukaryotic genes into mouse eggs will permit the molecular and genetic analysis of their integration and regulation during development and, eventually, their germ line transmission as new heritable elements.