We propose to generate, by gene targeting, mutant mice defective in cell lineage determination and pattern formation. Since early development in mouse closely parallels human early development, these mutants will provide models for studying causes of human birth defects as well as human infertility problems resulting from early embryonic wastage and spontaneous abortion. Gene targeting, the homologous recombination of DNA sequences residing in the chromosome with newly introduced DNA sequences, provides the means for specifically altering the mouse genome. By using mouse embryo-derived stem cells as the recipient cell line for the gene targeting experiments, the means are available for transferring the mutations generated in cell culture to living mice. The advantage of this scenario for creating specific developmental defective mice compared to random mutagenic methods is that on the one hand, the nature of the mutations at the discretion of the experimentor and on the other hand, unlike random mutagenic methods, the projected frequency of generating mice with specific mutations by gene targeting is sufficiently high to make the procedure practical. The particular mutant mice that we propose to generate will contain null mutations in the int-1, int-2 and the homeo box genes hox 1.2 and hox 1.3. This set of target genes was selected for our initial experiments because they should yield a spectrum of defects in early development where a correlation with altered cell lineage patterns should be evident. A genetic analysis of these mutants should provide a deeper understanding of the roles of these genes in controlling pattern formation. In the course of these pioneering experiments, a major effort will also be devoted towards developing practical strategies for generating mice of any desired mutant genotype.