Chem-seq can be used to detect the binding of small-molecule ligands, such as therapeutic drugs, to proteins associated with the genome. This information may provide important insights into the perturbation of cellular function by small-molecule drugs. The Chem-seq method uses 2 approaches. In living cells, the biotinylated drug is added to allow drug-target binding. The complex is crosslinked with formaldehyde, the cells are lysed and sonicated, and the complex is captured on streptavidin beads. The enriched DNA fragments are purified and sequenced. For in vitro analysis, the biotinylated drug is added to a cell extract, and the remaining steps are performed as for the in vivo procedure.