ERBB2 and breast cancer: Regarding Ki-67 labeling index according to HER2 status, patients with low Ki-67 labeling index were more frequent in the HER2-low group than the HER2-IHC 0 group for both HR-positive breast cancer and TNBC, regardless of the Ki-67 cutoff value (P = 0.001 and 0.001, respectively).