We found that (1) 854 (26.9%) of the 3,169 women in our series previously classified as having luminal A–like breast cancer (using the threshold value <14% Ki-67 expression) would have been reclassified as having luminal B–like breast cancer and, consequently, (2) most should have been treated with cytotoxic drugs. Here, MKI67 is linked to breast carcinoma.