e12535 Background: Categorical combinations of ER, PR, HER2, and Ki67 levels are traditionally used to classify patients into luminal A and B-like subtypes in order to inform treatment choice. Accounting for nearly 70% of all breast malignancies, luminal cancer is heterogeneous, harboring subtypes with distinct molecular profiles and clinical outcomes. Although most patients with luminal-type disease respond well to endocrine therapy alone, some develop recurrences benefiting from additional cytotoxic therapy. Identifying such cases a priori remains a challenge but would enable patients to be spared the debilitating side-effects of ineffective chemotherapy. In this regard, the efficacy of chemotherapy and disease recurrence relate to (i) ER driven G1/S perturbations and/or (ii) quiescent cell populations arrested in the G0/G1 phase of cell cycle. This study aimed to develop a histopathology whole slide image (WSI)-based, low cost, rapid and automated approach to: (i) predict ER/PR/Ki67 status, (ii) quantify quiescence burden, (iii) develop a G1/S-based patient stratification system for luminal A/B patients, and (iv) achieve a quiescence burden-based stratification of TNBC patients. Methods: This investigation centered on the initial clinical validation of a novel, immunostaining-free technology which uses information extracted from pre-treatment hematoxylin and eosin (H&E) stained slide WSIs alone to achieve these aims. Unlike conventional artificial intelligence-based approaches, the underlying proprietary algorithm and its prediction criteria are based on deterministic, hard-coded observational relationships of continuous scales drawn from WSI morphological features. In this instance, these represent tumor-related biological pathway disruptions and mitotic checkpoint perturbations, where G1/S perturbations enable luminal subtype stratification, and G0/G1 perturbations reflect quiescence burden. Back projecting the algorithm’s quiescence burden output on to the original WSIs enables morphological patterns to be mapped to quiescence burden.