ERBB2 and breast cancer: To analyze the effect of BET-PROTACs on HER2+ breast cancer, we first evaluated the efficacy of two well-known BRD4 degraders, MZ1 (based on the BETi JQ1) and ARV-825 (based on the BETi OTX015) in the HER2-overexpressing breast cancer cell lines BT474 and SKBR3.