One major postgenomic challenge is to characterize the epigenomes that control genome functions. The epigenomes are mainly defined by the specific association of nonhistone proteins with chromatin and the covalent modifications of chromatin, including DNA methylation and posttranslational histone modifications. The in vivo protein-binding and chromatin-modification patterns can be revealed by the chromatin immunoprecipitation assay (ChIP). By combining the ChIP assays and the serial analysis of gene expression (SAGE) protocols, we have developed an unbiased and high-resolution genome-wide mapping technique (GMAT) to determine the genome-wide protein-targeting and chromatin-modification patterns. GMAT has been successfully applied to mapping the target sites of the histone acetyltransferase, Gcn5p, in yeast and to the discovery of the histone acetylation islands as an epigenetic mark for functional regulatory elements in the human genome.