PDTEs were established from brain metastatic tissue from 3 breast cancer patients: T347 (ER+/HER2− primary breast to ER+/HER2 amplified in BCBM), T638 (ER+/HER2− primary breast tumor to ER+; gained HER2 expression in BCBM, HER2 non-amplified), T328 (ER+/HER2− in both primary breast and brain metastatic tumors) and from independent pleural/lung metastatic material in 2 of the samples HCI05 (ER+/HER2+) and HCI-011 (ER+/HER2−), all expanded in the mammary fat pad. The gene discussed is ERBB2; the disease is breast cancer.