ESR1 and breast cancer: Using the ratio of non-synonymous to synonymous mutations, 21 potential driver genes were identified in metatstatic breast cancers, including the key drivers TP53, PIK3CA, ESR1, GATA3, and KMT2C; as expected, TP53 was enriched in TNBC, whereas ESR1, PIK3CA, and GATA3 were more frequently mutated in ER+ metastatic breast cancers [47].