ERBB2 and breast cancer: Futhermore, carfilzomib and bortezomib markedly decrease the phosphorylation status of a constitutively active HER2 mutant that is resistant to trastuzumab and lapatinib [40], suggesting that proteasome inhibitors could be an interesting therapeutic opportunity for breast cancer patients with mutated HER2 variants that are resistant to commonly used anti-HER2 drugs.