Breast and ovarian cancer susceptibility gene 1 (BRCA1) was identified from the genomic analysis of breast cancer families. Women heterozygous for a germ line mutation in the BRCA1 gene have a high risk of developing breast or ovarian cancers. BRCA1-mutant mice were created to study the function of BRCA1, but the heterozygous mice were healthy. The reason for this difference in phenotype between mice and humans is unknown. Here, I emphasize that the estrous cycle of mice differs from the menstrual cycle of humans and that this difference may be responsible for the lack of tumorigenesis in the BRCA1-mutant mouse model. Even if this difference in the cycles is not responsible for the phenotype difference, researchers should note this estrous cycle variation in reports of mouse BRCA1 studies. A method for reducing the influence of the short estrous cycle in mouse studies is also discussed in this text.
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