BRCAl is proposed to be a tumor suppressor gene. To explore the biological function of BRCAl, a partial deletion (amino acids 300-361) of mouse Brcal exon 11 was introduced into the genome of embryonic stem (ES) cells by homologous recombination. Mice carrying one mutated allele of Brcal appear normal and are fertile up to 10 months of age without any sign of illness. However, no viable progeny homozygous for the Brcal mutant allele were obtained. Detailed analysis of large numbers of embryos at different stages of development indicated that the homozygous mutant concepti are severely retarded in growth as early as embryonic day 4.5 (E4.5) and are resorbed completely by E8.5. Although the homozygotes at E5.5-E6.5 are able to synthesize DNA and display distinguishable embryonic and extraembryonic structures, they fail to differentiate and form egg cylinders. Consequently, they were unable to form primitive streaks and undergo gastrulation. Consistent with these in vivo results, blastocysts homozygous for mutated Brcal alleles are at a considerable disadvantage when grown in vitro. These observations suggest that Brcal has an important role in the early development of mouse embryos.