ERBB2 and breast cancer: Similar to previous studies [2,6], the Ki-67 level in HER2-low tumor was significantly lower than that in HER2-zero or HER2-positive tumors (median Ki67 index, 30% in HER2-low vs 50% in HER2-zero vs 40% in HER2-positive tumors, p = 0.010), suggesting a slower proliferation rate of HER2-low breast cancer.