Early in the summer of 1991, Valerio Orlando, a postdoc in Renato Paro's lab at the University of Heidelberg, began working on the problem of identifying where proteins bind to chromatin in vivo. Researchers had already figured out how to determine whether a particular protein bound to a specific sequence in vitro. But what about protein occupancy in a living cell?
Orlando and Paro's idea was simple: Crosslink protein-DNA complexes using formaldehyde, immunoprecipitate with an antibody to the protein of interest, reverse the crosslink to release the purified sequences, and then use that DNA to probe a Southern blot of potential binding sites. But the technique, called chromatin immunoprecipitation (ChIP), wasn't working very well, Paro says. "After the crosslink and immunoprecipitation, we realized we would only have very little DNA material left, and this would not be sufficient to do Southern hybridization." So on June 1, 1992, the pair discussed ...
