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Mitochondria are critical regulators of energy production, metabolism and cell death. Warburg observed that in the presence of oxygen, many cancer cells take up abundant amounts of glucose, and excrete the carbon mainly as lactate. Warburg postulated that this aerobic glycolysis was essential and universal in cancer, and was caused by mitochondrial dysfunction. Yet, more recent findings suggest that aerobic glycolysis also occurs in rapidly dividing normal cells to support anabolic reactions needed for cell growth and replication. Furthermore, a number of metabolites generated by the tricarboxylic acid cycle and the electron transport chain are also used for macromolecular synthesis in proliferating cancer and normal cells. This suggests that an increase in mitochondrial activity, especially those functions related to anabolic metabolism, may be required for cancer development and progression. One mechanism to increase mitochondrial function is through mitochondrial biogenesis, which is partly regulated by regulating mitochondrial DNA (mtDNA) copy number. Alterations in mtDNA sequence and copy number have been implicated in various cancers. Since most prior measurements of mtDNA copy number have been performed using homogenates of complex tissues composed of multiple cell types, very little is known about cell type-specific mtDNA copy number heterogeneity. Here, we validated a quantitative in situ hybridization approach to study mtDNA levels at the single cell level in tissue sections. In hierarchically organized self-renewing tissues, higher levels of mtDNA were observed in stem/proliferative compartments than in differentiated compartments. Also, we found zonal heterogeneity of mtDNA levels in the liver that mirrors the known oxygen tension gradient. In the kidney, we observed markedly lower mtDNA levels in glomeruli and the collecting ducts compared to cortical proximal and distal tubules. This work provides visualization of mtDNA copy number in distinct cell populations in situ for the first time across normal mammalian tissues, and can serve as a foundation for quantifying cell type-specific mtDNA copy number dynamics in any relevant diseases, especially in cancers and their precursor lesions. Citation Format: JIAYU CHEN, Qizhi Zheng, Lauren B. Peiffer, Jessica L. Hicks, Michael C. Haffner, Avi Z. Rosenberg, Moshe Levi, Xiaoxin Wang, Busra Ozbek, Srinivasan Yegnasubramanian, Angelo M. De Marzo. An in situ hybridization assay reveals high mitochondrial DNA copy number in stem cell/proliferative cell compartments [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4790.