The breast and ovarian cancer susceptibility gene, BRCA1, has been cloned and shown to encode a zinc-finger protein of unknown function1. Mutations in BRCA1 account for at least 80% of families with both breast and ovarian cancer, as well as some non-familial sporadic ovarian cancers^2,3. The loss of wild-type BRCA1 in tumours of individuals carrying one nonfunctional BRCA1 allele suggests that BRCA1 encodes a tumour suppressor^4–6 that may inhibit the proliferation of mammary epithelial cells⁷. To examine the role of BRCA1 in normal tissue growth and differentiation, and to generate a potential model for the cancer susceptibility associated with loss of BRCA1 function, we have created a mouse line carrying a mutation in one Brcal allele. Analysis of mice homozygous for the mutant allele indicate that Brcal is critical for normal development, as these mice died in utero between 10 and 13 days of gestation (E10–E13). Abnormalities in Brcal -deficient embryos were most evident in the neural tube, with 40% of the embryos presenting with varying degrees of spina bifida and anencephaly. In addition, the neuroepithelium in Brca 1-deficient embryos appeared disorganized, with signs of both rapid proliferation and excessive cell death.