Mutations affecting early development of the mouse embryo will be generated using gene traps in embryonic stem (ES) cells. In this approach, a reporter gene is introduced into ES cells by a retrovirus, but expression of the reporter can only originate from the promoter of a gene into which it has inserted. Since the reporter gene used in these experiments encodes beta-galactosidase activity, expression of the mutated gene can be conveniently followed in heterozygous mice. The consequences of gene disruption can be assessed by breeding the mice to homozygosity. In previous work, 42 lines of mouse mutants have been generated, which display a diversity of beta-galactosidase expression pattern during development. A high proportion of overt phenotypes was observed among these strains, since provirus insertion leads to embryonic lethality in 18 strains and to male sterility in two others. In one lethal strain, the provirus has disrupted the gene which encodes the transcription factor TEF1, leading to cardiac and central nervous system defects at midgestation. We will continue to analyze these strains by histology to understand the defects in mutant embryos, and by cloning the mutated gene. We will also use gene traps to identify genes induced during development in response to two types of factors thought to play critical roles in early development: retinoic acid, a factor believed to have the properties of a morphogen; and growth factors, whose receptors are protein tyrosine kinases implicated in several classical mouse mutations affecting early mouse development. It is anticipated that these studies will lead to the identification of genes playing key roles in early mouse development, and help identify, using genetics, critical components in specific signaling pathways.