However, we did not see a significant difference in age at diagnosis between first ER+ breast cancers with and without loss of wt BRCA1. In addition all 10 ER+ second cancers that developed in BRCA1 carriers demonstrated loss of wt BRCA1. It is possible that these findings are due to limited numbers, but it is also possible that some mechanism other than incidental development of breast cancer, with functioning wt BRCA1, is needed to explain why the development of ER+ breast cancers is more common as BRCA1 mutation carriers age. This evidence concerns the gene BRCA1 and breast cancer.