e12534 Background: In hormone receptor-positive, HER2-negative early-stage breast cancer (HR+/HER2- EBC), the biological characteristics and therapeutic response to endocrine treatment are fundamentally shaped by the synergistic interplay between estrogen receptor (ER) and progesterone receptor (PR). Nevertheless, the role of discordant expression levels between ER and PR—termed ΔER-PR—remains poorly studied, and its effect on patient outcomes is unclear. Methods: This large-scale, multicenter r