ERBB2 and breast carcinoma: On the other hand, the linkage between ERα-related cell proliferation/survival and FASN signaling is such that tamoxifen-unresponsive FASN overexpression in endocrine-resistant ER/HER2-positive breast cancer cells might reflect the constitutive hyperactivation of the downstream (PI3K/AKT and RAS/MAPK) signaling.