To further corroborate the suggestion that EVOO polyphenols induce cell growth inhibition by depleting HER2 protein in HER2-overexpressing breast cancer cells via proteasomal degradation, we analyzed whether supra-additive or synergistic interactions occurred upon concurrent treatment with EVOO polyphenols and anti-HER2 agents that promote HER2 down-regulation through proteasome-independent mechanisms [12,46,47]. This evidence concerns the gene ERBB2 and breast cancer.