ESR1 and breast cancer: Invasive lobular breast carcinomas (ILC) account for up to 15% of all invasive breast cancer (BC) cases and represents the second most frequent histological subtype after invasive ductal BC (IDC), the latter also being formally referred to as invasive breast carcinoma of no special type.1 ILCs typically express the estrogen receptor (ER, coded by the ESR1 gene) and lack HER2 amplification.