In our study of early-stage breast cancer patients with favorable prognosis—women aged 50 years or older, with node-negative disease and ER positive, PR positive, HER2− tumors—we found that visual assessment of Ki67 IHC after undergoing IKWG training was moderately to strongly reproducible across two IHC pathologists (ICC = 0.82), with 8.8–16.0% having Ki67 scores ≤ 5% and 11.4–13.4% having scores ≥ 30%. Here, ERBB2 is linked to breast cancer.