A critical step of drug discovery is the establishment of robust cellular and molecular assays to test drug effects and specificity.Chromatin immunoprecipitation (ChIP) is a robust technique for studying protein-DNA interactions within the cell.ChIP consists in (i) preparation of chromatin from cells,(ii) selective immunoprecipitation,from the chromatin preparation,of a DNA-binding protein of interest using specific antibodies,(iii) isolation of the co-precipitated DNA and (iv) identification of the precipitated sequences by PCR,sequencing or hybridization of microarrays (ChIP-on-chip).ChIP assays have been used to map the location of post-translationally modified histones,transcription factors and chromatin modifying enzymes in restricted genomic regions and at the genome-wide level.