Co-immunoprecipitation is a popular technique for protein interaction discovery. Co-IP is conducted in essentially the same manner as an IP, except that the target antigen precipitated by the antibody is used to co-precipitate its binding partner(s) or associated protein complex from the lysate. The assumption that is usually made when associated proteins are co-precipitated is that these proteins are related to the function of the target antigen at the cellular level. This is only an assumption, however, that is subject to further verification.