ESR1 and breast cancer: Based on unique population-based data from Iceland where 85% of patients had been tested for the Icelandic BRCA2 founder mutation we show here that young women with breast cancer shared the abovementioned unconventional prognostic associations with BRCA2 carriers even though virtually no BRCA mutation carriers were included in the young group, rendering it unlikely that the worse outcomes associated with an ER+ status could be explained by the high prevalence of germline BRCA mutations in young breast cancer patients.