Although mutation of ATR alone is not enriched in either ER+/HER2− or TNBC, we found twofold enrichment for comutation of ATR and TP53 in ER+/HER2− breast cancer (36% ATR/TP53 comutated versus 18% TP53 mutation alone, P = 0.002) (Fig. 1C). Here, ESR1 is linked to breast cancer.