ESR1 and neoplasm: To disentangle the contributions of ER and PR to tumor growth and metastatic spread, we recurred to using the particularly E2-sensitive and readily manipulatable MCF7 cells and lentivirally transduced them with shCTRL or shRNAs targeting either ESR1 or PGR. Following drug selection, some cells were seeded for in vitro colony formation, and others xenografted intraductally.