Inserm U1280, Gif-sur-Yvette 1 article published in JoVE Biology Visualization and Quantification of Endogenous Intra-Organelle Protein Interactions at ER-Mitochondria Contact Sites by Proximity Ligation Assays Vera Filipa Monteiro-Cardoso1,2, Romain Le Bars1,3, Francesca Giordano1,2 1Institute for Integrative Biology of the Cell (I2BC), CEA, CNRS, Université Paris-Saclay, 2Inserm U1280, 3Imagerie-Gif, Light Microscopy Facility, Institute for Integrative Biology of the Cell (I2BC), CEA, CNRS, Université Paris-Saclay The need for new approaches to study membrane contact sites (MCSs) has grown due to increasing interest in studying these cellular structures and their components. Here, we present a protocol that integrates previously available microscopy technologies to identify and quantify intra-organelle and inter-organelle protein complexes that reside at MCSs.