ESR1 and breast cancer: Hierarchical cluster analysis showed that the pure mucinous tumors are more similar to the ER+ breast cancer cells, while the E2 treated tumor regions (either MUCm or ILC) cluster within the ER- breast cancer cells (despite the fact the tumors retain ER and PR13); this is likely driven by the high proliferation rates of the ILC region of BCK4 tumors (Fig. 2b).