Singapore General Hospital 1 article published in JoVE Biology Co-immunoprecipitation Assay Using Endogenous Nuclear Proteins from Cells Cultured Under Hypoxic Conditions Xiaofeng Zheng1,2, Calvin Qing Wei Ho1, Xiaowei Zheng3, Kian Leong Lee4, Katarina Gradin5, Teresa S. Pereira3, Per-Olof Berggren1,2,3, Yusuf Ali1,2 1Lee Kong Chian School of Medicine, Nanyang Technological University, 2Singapore Eye Research Institute (SERI), Singapore General Hospital, 3The Rolf Luft Research Center for Diabetes and Endocrinology, Karolinska Institutet, Karolinska University Hospital, 4Cancer and Stem Cell Biology Program, Duke-NUS Medical School, 5Department of Cell and Molecular Biology, Karolinska Institutet Here we describe a co-immunoprecipitation protocol to study protein-protein interactions between endogenous nuclear proteins under hypoxic conditions. This method is suitable for demonstration of the interactions between transcription factors and transcriptional co-regulators at hypoxia.