IntroductionIntimately controlled, complex biological networks are responsible for the functioning of a multicellular organism. These controls operate at various levels, including cell-cell communication and coordination as well as intracellular regulation. Regulation also must occur at the level of the gene. The reductionist approach suggests that understanding gene regulation will help to understand yet unsolved biological problems such as morphogenesis, cell differentiation, and cell growth in mammalian development.The advent of gene cloning as a routine method has facilitated analysis of eukaryotic gene regulation through powerful techniques such as introduction of viruses or cloned genes into mammalian tissue-culture cells, as reviewed by Pellicer et al. (1980a). Because of the limitations of such in vitro studies for analyzing tissue-specific regulation of gene expression, attempts were first made to introduce viruses as model genes, and then cloned genes, into the mammalian embryo. The major rationale underlying this approach is that regulation and function of the gene can be studied in vivo, where it will be exposed from the onset of development to all possible regulatory factors. This should permit rigorous analysis of gene expression in various somatic cells of the adult organism. If the gene is also present in the germ cells, the stability and inheritance of expression can be studied in subsequent generations of animals carrying the gene.With the development of successful methods for introducing genes into mouse embryos, other areas of research have also been opened up.