One clue may come from observations concerning loss-of-heterozygosity of the wild-type allele in BRCA1 carriers, which is observed at a similar rate in ER-positive and ER-negative tumours, thereby suggesting a similar degree of impairment to the homologous recombination pathway.15,16 Clinical data to address this question are limited, but three olaparib studies reported outcomes in patients with ER-positive BC. This evidence concerns the gene BRCA1 and breast cancer.