It received “Breakthrough Therapy” designation from the FDA after the open-label phase II PALOMA-1 trial demonstrated a statistically significant improvement in PFS when palbociclib was added to letrozole in the treatment of postmenopausal women with metastatic ER+/HER2-breast cancer who had not previously received any systemic treatment for their advanced disease [15]. Here, ERBB2 is linked to breast cancer.