A wide array of mutations now numbering more than 200 have been identified in the BRCA1 gene, one of the two breast cancer susceptibility genes identified so far. In addition, there have been several variants described but it is not known if they really represent functionally significant mutations of the BRCA1 gene. We report evidence to show that the duplication/ insertion of 12 base pairs in intron 20 could have a real effect on expression of the BRCA1 gene, although it was also present in 1% of our control population.