Samples from 646 invasive, non-metastatic primary ER-positive BC patients who did not receive any systemic endocrine therapy as standard of care treatment, allowed us to study the pure prognostic impact of A3B protein expression on disease-free survival (DFS), metastatic disease-free survival (MFS), breast cancer specific-survival (BCSS), and overall survival (OS). This evidence concerns the gene APOBEC3B and breast cancer.