We found that the GPD1 expression level has significant prognostic value for breast cancer patients with ER-positive tumours (for NM, ER+, AE: p = 0.0027, HR = 0.91, 95% CI = 0.85–0.97, NP = 3,917, NE = 1,248 and for NM, ERM, AE: p = 0.0005, HR = 0.91, 95% CI = 0.86–0.96, NP = 5,488, NE = 1,854) (Supplementary Tables 3–4 and Supplementary Figure 3). This evidence concerns the gene ESR1 and breast cancer.