In patients with primary HER2-positive and hormone receptor-positive BC, patients with diabetes who did not receive metformin treatment had lower rates of disease-free survival (DFS) (multivariable HR, 1.40; 95% CI, 1.01–1.94; p = 0.043) and OS (multivariable HR, 1.87; 95% CI, 1.23–2.85; p = 0.004) than patients with diabetes who had received metformin treatment (Sonnenblick et al., 2017). Here, ERBB2 is linked to diabetes mellitus.