The detection of circulating tumor cells (CTCs) in the peripheral blood of metastatic breast cancer (MBC) patients is an independent marker of prognosis. This large prospective multicenter study aimed to assess the impact of CTCs on overall survival (OS) and progression free survival (PFS) in patients with predefined molecular subgroups of MBC. To this end, 468 MBC patients were divided into three subgroups based on immunohistochemical staining of the primary tumor: (1) hormone receptor-positive/HER2-negative (HorR+/HER2-), (2) HER2-positive (HER2+), and (3) HorR-negative/HER2-negative (HorR-/HER2-) patients. CTC status (
