To determine whether MCM3 had a predictive potential in addition to its prognostic value, the association of MCM3 expression and clinical outcome was investigated in 2 cohorts of ER+ breast cancer patients that did not receive adjuvant endocrine or chemotherapy; one based on gene array data obtained from KMplot.com and the second based on immunohistochemical evaluated of this subgroup of the STO-3 cohort. The gene discussed is ESR1; the disease is breast cancer.