Briefly, LUMINA is a multicentre, single-arm prospective cohort study investigating the clinical value of clinicopathological characteristics together with Ki-67 immunohistochemistry-based phenotyping for identifying women ≥ 55 years with sufficiently low-risk molecularly defined T1N0 luminal A breast cancers (ER ≥ 1%, PR > 20%, HER2 negative, and Ki-67 ≤ 13.25%) who can be adequately treated with breast-conserving surgery and endocrine therapy alone without compromising oncological outcomes. Here, ESR1 is linked to breast cancer.