We developed a strategy to introduce epitope tag-encoding DNA into endogenous loci by homologous recombination-mediated 'knock-in'. The tagging method is straightforward, can be applied to many loci and several human somatic cell lines, and can facilitate many functional analyses including western blot, immunoprecipitation, immunofluorescence and chromatin immunoprecipitation-microarray (ChIP-chip). The knock-in approach provides a general solution for the study of proteins to which antibodies are substandard or not available.