In our cohort of 357 patients with high-grade ductal carcinoma in situ (DCIS), by applying the 2021 IKWG recommendations, we identified 81 cases (23%) with a Ki67 value between a < 1–5% interval, while 49 (14%) cases had a value above 30%. This evidence concerns the gene MKI67 and ductal breast carcinoma in situ.