ESR1 and breast cancer: Moreover, analysis of public data profiling gene expression in breast cancer patients after blocking ERα using the aromatase inhibitor letrozole [41] found an increase in EGR1 transcript abundance (p = 1.775e-06, Wilcoxon rank sum test; Fig 5F), suggesting that the interaction between ERα and EGR1 is active in breast cancer tissue, though we observed no change in GDNF.