ESR1 and breast cancer: For the functional validation, we used HP5008 cells, which are mammary tumor cells derived from Fgfr2-S252W;MMTV-Cre mice with a single spontaneously arising mammary tumor at the age ~12 months [25], showed positive for ERα but negative for PR and HER2 expression in all tumor samples when injected into nude mice (Fig. S3).