A third Cox regression model focused on ER+ breast cancer patients revealed that the ER+/HER2+ subtype (HR = 1.81, 95% CI: 1.09–2.96, p = 0.02) and stage IV disease at diagnosis (HR = 1.83, 95% CI: 1.1–3.18, p = 0.03) were significant predictors of shorter time to BM diagnosis in the unadjusted model. This evidence concerns the gene ESR1 and breast cancer.