ESR1 and neoplasm: To investigate how the HR+ PDOs respond to treatment regimens used in the patients and if they respond similarly to the original tumor, we treated five HR+ PDO lines (#7, #8, #10, #26, and #27) with an ER-targeting agent (fulvestrant), or a CDK4/6 targeting agent (palbociclib), or with the combination (Fig. 4, B and C).