ESR1 and breast cancer: Other factors besides TP53 can modulate genomic instability and AI resistance, and genomic instability is significantly inversely correlated with the average expression of the ER-regulated genes TFF1, GREB1, PGR, and PDZK1 in ER-positive tumors from the Molecular Taxonomy of Breast Cancer International Consortium24 (r = −0.24; P < .001, Pearson’s correlation), which suggests that other factors besides ER are driving proliferation and resistance to AI in tumors with high genomic instability.