ESR1 and breast cancer: Risk factors for contralateral breast cancer (CBC) include carrying a BRCA1 or BRCA2 mutation 10, 11, 12, 13, having a family history of breast cancer 14, 15, 16, 17, 18, being diagnosed with an estrogen receptor (ER)‐negative first primary breast cancer 19, 20 or one with lobular histology 21, 22, and reproductive factors 23.