To test whether the mitotic arrest associated with induction of cyclin B1 suppresses BRCA1-mutant breast cancer in vivo, we examined the effect of vinblastine in an allograft model of transplanted mammary tumor tissues from Brca1co/coMMTV-cre mice, which spontaneously develop mammary tumors starting at ~10 months of age. The gene discussed is BRCA1; the disease is breast cancer.