As with most other proteins, clock proteins physically interact with one another. Coimmunoprecipitation (coIP) is the most straightforward technique to study protein-protein interactions in vivo, if antibodies against the proteins of interest are available. To perform coIP, first an antibody against a target protein is coupled to Sepharose beads through protein A or G, then the complexes containing the target protein are immunoprecipitated with the antibody-coupled beads by centrifugation. Protein components in the complexes are visualized by Western blotting using antibodies specific to the different components.