ESR1 and breast cancer: GATA3, while not enriched in endocrine resistance, is an important marker of ER dependence and luminal identity (93)—GATA3muts were preferentially enriched in ER+HER2-low tumors over ER+HER2-0 (16% of 370 vs. 9% of 336) (26) while also present in ~18% (18/99) of HER2-low primary BC (2).