The assessment of BRCA1 and BRCA2 coding sequences to identify pathogenic 
mutations associated with inherited breast/ovarian cancer syndrome has provided 
a method to identify high-risk individuals, allowing them to seek preventative 
treatments and strategies. However, the current test is expensive, and cannot 
differentiate between pathogenic variants and those that may be benign. Focusing 
only on one of the two BRCA partners, we have developed a biological assay for 
haploinsufficiency of BRCA1. Using a series of EBV-transformed cell lines, we 
explored gene expression patterns in cells that were BRCA1 wildtype compared to 
those that carried (heterozygous) BRCA1 pathogenic mutations. We identified a 
subset of 43 genes whose combined expression pattern is a sensitive predictor of 
BRCA1 status. The gene set was disproportionately made up of genes involved in 
cellular differentiation, lending credence to the hypothesis that single copy 
loss of BRCA1 function may impact differentiation, rendering cells more 
susceptible to undergoing malignant processes.