Chromatin immunoprecipitation (ChIP) assay, is an experimental method used in molecular biology. The purpose of this assay is to determine whether a transcription factor binds to a particular region on the endogenous chromatin of living cells or tissues. The in vivo nature of this method is in contrast to other approaches traditionally employed to answer the same questions.
The principle underpinning this assay is that DNA-bound proteins (including transcription factors) in living cells can be cross-linked to the chromatin on which they are situated. This is usually accomplished by a gentle formaldehyde fixation. Once the proteins are immobilized on the chromatin, the whole protein-DNA complex can be immunoprecipitated using an antibody against the protein in question. The isolated protein/DNA fraction can then be purified for DNA. The identity of the DNA fragments isolated in connection with the protein can then be determined by PCR or, more recently, DNA microarray.