Simple SummaryBreast cancer continues to be the number one cause of cancer mortality among women. Because breast cancer is a heterogenous disease there is a need for a more personalized approach to treatment. Such an approach requires the understanding of the molecular root cause of each cancer and the identification of the responsible molecule(s) or pathway(s). SigmaR1 is a receptor implicated in certain types of breast cancer and represents a promising target for a new generation of personalized treatments. However, there is a need for understanding its precise cellular role in order to target it effectively. This article reviews the current knowledge about SigmaR1 in breast cancer biology and potential treatment and proposes a new model as to how SigmaR1 operates within the cell in order to devise new effective ways for utilizing it in the clinic.AbstractDespite the major progress in treating breast cancer, recurrence remains a problem and types such as triple-negative breast cancer still lack targeted medicine. The orphan Sigma receptor1 (SigmaR1) has emerged as a target in breast cancer, but its mechanism of action is unclear and hinders clinical utility. SigmaR1 is widely expressed in organ tissues and localized to various sub-cellular compartments, particularly the endoplasmic reticulum (ER), the mitochondrial-associated membranes (MAMs) and the nuclear envelope. As such, it involves diverse cellular functions, including protein quality control/ER stress, calcium signaling, cholesterol homeostasis, mitochondrial integrity and energy metabolism. Consequently, SigmaR1 has been implicated in a number of cancers and degenerative diseases and thus has been intensively pursued as a therapeutic target. Because SigmaR1 binds a number of structurally unrelated ligands, it presents an excellent context-dependent therapeutic target. Here, we review its role in breast cancer and the current therapies that have been considered based on its known functions. As SigmaR1 is not classified as an oncoprotein, we propose a model in which it serves as an oligomerization adaptor in key cellular pathways, which may help illuminate its association with variable diseases and pave the way for clinical utility in personalized medicine.