For breast cancer patients, the detrimental effect of synchronous brain metastases on OS was highest in HER2+ and HR− patients (HR = 2.47 [95% CI: 2.02–3.01]), followed by HER2− and HR+ patients (HR = 2.23 [95% CI: 2.01–2.47]), triple-negative patients (HR = 2.00 [95% CI: 1.75–2.29]), and HER2+ and HR+ patients (HR = 1.93 [95% CI: 1.62–2.31]; P < .001). This evidence concerns the gene ERBB2 and breast cancer.