INTRODUCTIONThis protocol describes the use of chromatin immunoprecipitation technology (ChIP) to analyze interactions of proteins or protein complexes with DNA in vivo. In this approach, the material is fixed with formaldehyde to preserve DNA-protein and protein-protein associations, the cells are lysed, and the chromatin is cut and solubilized. The chromatin suspension is immunoprecipitated with an antibody against the protein(s) of interest, and the coimmunoprecipitated DNA fragments are analyzed. The following protocol has been established for the cultured cell line Schneider 2 (S2) from Drosophila melanogaster. If other tissue is used, certain steps of the protocol may need to be optimized; the main variation is likely to be in the cross-linking step.