Abstract Estrogen receptor (ER) + breast cancer (BC) comprises over 70% of BC cases and are targeted via ER modulated therapies. Despite this, ER+BC patients can experience recurrence within 20 years and the majority of BC related deaths can be attributed to metastatic ER+BC. These distant metastases are commonly diagnosed as endocrine therapy resistant. Thus, there is an unmet need to identify novel biomarkers for treating ER+ patients with metastases. We have identified a tumor suppressor gene, singleminded 2s (SIM2s), expressed in breast epithelial cells that inhibits epithelial to mesenchymal transition and metastasis, and is downregulated in the progression of breast disease. In ER+BC cell lines, loss of SIM2s results in upregulation of mesenchymal markers and increased PI3K/Akt signaling. Dysregulation of the PI3K/Akt signaling pathway in ER+BC is involved with tumor progression and acquired therapeutic resistance. Our study suggests loss of SIM2s confers resistance in ER+BC through a PI3K subunit switch resulting in upregulation of pro-survival signaling. MCF7 SIM2 knock out cells exhibit increased expression of mesenchymal markers and undergo a phenotypic change compared to wild type cells. Utilizing a migration/invasion assay, MCF7 SIM2 knock out cells exemplify an increase in invasion potential compared to wild type cells. Furthermore, changes in PI3K subunit expression were observed via western blot and real time qPCR analysis. Clonogenic assays revealed an acquired resistance to PI3Kα inhibition, but a susceptibility to PI3Kδ inhibition. This is a significant finding as a current standard of care for patients with ER+ breast cancer recurrence is PI3Kα inhibition. PI3Kδ is an already approved therapeutic target in chronic lymphocytic leukemia, thus PI3Kδ may present a new therapeutically targetable opportunity for ER+BC recurrence. Elucidating the mechanism for acquired therapeutic resistance is an integral avenue for understanding how breast cancer progresses and improving the prognosis of ER+BC patients. Citation Format: Garhett Wyatt, Rachel Steinmetz, Traci Lyons, Weston Porter. LOSS OF SINGLEMINDED 2S RESULTS IN A PI3K SUBUNIT SWITCH WHICH DRIVES THERAPEUTIC RESISTANCE IN ESTROGEN RECEPTOR POSITIVE BREAST CANCER [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-05-08.