BRCA1 and breast cancer: In the case of hereditary BRCA1/2-related breast cancers, the frequency of loss of heterozygosity or allelic imbalance (LOH/AI) was found to be approximately equal in the mammary epithelium (59.7%) and the adjacent stroma (66.2%), whereas a higher frequency of LOH/AI was observed in mammary epithelium (36.7%) compared to the stromal compartment (28.4%) in sporadic breast cancers [66].