Ki-67 expression is routinely evaluated in many neoplasms as a measure of proliferation activity, and in some cases, such as human epidermal growth factor receptor 2 (HER2)-negative luminal breast cancers, it can be used to identify which patients shall be treated with adjuvant chemotherapy.31–33 However, the usefulness of Ki-67 for USMTs is controversial. The gene discussed is ERBB2; the disease is breast cancer.