Since CHEK2 c.1100delC mutation carriers as well as patients with somatic TP53 mutations have been shown to have unfavorable survival [5, 8, 10, 11, 46–49], we evaluated this among our retrospective cohorts of 760 lymph-node negative systemic treatment-naïve ER+ BC patients (prognostic cohort) and 323 hormone-naïve ER+ BC patients treated with first-line tamoxifen for recurrent disease (predictive cohort; clinicopathological variables in Additional file 1: Tables S10-11). This evidence concerns the gene CHEK2 and breast cancer.