Co-IP Protocol Co-Immunoprecipitation (Co-IP) is an extension of immunoprecipitation (IP) with which Co-IP shares the same fundamental principle of the specific antigen-antibody reaction. By targeting a known protein with a specific antibody, it may be possible to pull the entire protein complex out of solution, thereby identifying unknown members of the complex. However, Co-IP requires greater care and more physiologically relevant conditions than traditional IP. When successful, Co-IP pulls down not only the protein of interest but also its interaction partners. Thus, Co-IP is a powerful technique to identify protein complexes and determine protein-protein interactions. Here we provide a detailed procedure for Co-IP to study protein–protein interactions. Harvest and Wash Cells：Transfer the cultured cells from the culture dish to a 15-mL conical tube. Centrifuge at 500×g for 2 min at 4°C and remove the supernatant. Wash with ice-cold PBS and centrifuge at 500×g for 2 min at 4°C. Remove the supernatant. Repeat Step 3 twice.