ERBB4 and breast cancer: Apart from BTK inhibition, ibrutinib also blocks the activation of EGFR, human epidermal growth factor receptor 2 (HER2), Erb-B2 receptor tyrosine kinase 3 (ErbB3), and Erb-B2 receptor tyrosine kinase 4 (ErbB4), which results in increased apoptosis of breast cancer cells.