In order to generate an in vitro mouse model for the study of human ovarian cancers, we compared the effects of a truncated Brca1 mutant expression on cellular phenotype with those of a full-length sense and antisense Brca1 expression in the ID-8 mouse epithelial ovarian cancer cell line. The examined cellular processes include proliferation, tumorigenicity in syngeneic mice in vivo and sensitivity/resistance to several cytotoxic drugs. We found that the expression of a spontaneous truncated Brca1 mutant in ID-8 cells which contain two endogenous wild-type Brca1 alleles led to a dominant-negative effect of Brca1, demonstrated by an increase in tumorigenicity in vivo and in chemosensitivity. Expression of a truncated Brca1 mutant in a mouse epithelial ovarian cancer cell line could thus provide a powerful in vitro model for the study of human BRCA1-related ovarian tumorigenesis.