Chromatin immunoprecipitation (ChIP) offers an advantageous tool for studying protein-DNA interaction. With ChIP, the experimenter can determine if a specific protein binds to the specific sequences of a gene in living cells by combining it with PCR (ChIP-PCR), microarray (ChIP-chip), or sequencing (ChIP-Seq) techniques. For example, the measurement of the amount of methylated histone H3 at lysine 9 (H3K9me) associated with a specific gene promoter region under various conditions can be achieved through a ChIP-PCR assay, while recruitment of H3K9me to the promoters on a genome-wide scale can be detected by ChIP-chip. In particular, a ChIP method with specific antibodies directly against various transcriptional factors is widely in demand.