The development of chromatin imsmunoprecipitation methods coupled with DNA microarray (ChIP-chip) technology has enabled genome-wide identification of cis-DNA regulatory elements to which transcription factors bind. Nonetheless, the ChIP-chip technology requires antibodies with extremely high affinity and specificity for the target transcription factors. Unfortunately, such antibodies are not available for most human transcription factors. In principle, this problem can be circumvented by utilizing ectopically expressed epitope-tagged proteins recognizable by well-characterized antibodies. However, such expression is no longer endogenous. To surmount this problem, we have successfully developed a facile method to knock in a 3xFlag epitope into the endogenous gene loci of transcription factors. The knock-in approach provides a general solution for the study of proteins for which antibodies are substandard or not available.