ERBB2 and heart failure: Studies have shown that murine ErbB2 expression is decreased to almost 99% in the heart at postnatal day 7 and further decline at P28.20 Besides, another study has shown a severe decrease (95%) in ErbB2 in the hearts of mice at 12 weeks of age.21 For example, deleting ErbB2 in knockout mice or treatment with ErbB2 inhibitor herceptin (trastuzumab) causes heart failure.3,22 Nrg1 signaling in the adult heart is mediated by ErbB2/ErbB4 heterodimers,23 which modulate cardiomyocyte contractile functions.22