ESR1 and neoplasm: To determine whether tumor cells produce soluble factors that activate STAT5 in vitro, conditioned medium (CM) samples were collected from estrogen receptor positive (ER +) (T47D, MCF7, BT-474, ZR751), human epidermal growth factor receptor 2 (HER2 +) (BT-474, SKBR3), and TNBC (Hs578T, MDA-MB-231, MDA-MB-468, BT-549) cells.