Chromatin proteins are involved in DNA transcription, replication, and repair, and a study of the binding sites of chromatin can provide insight into these roles. Methods used to determine chromatin-binding sites were cumbersome and error prone until Bas van Steensel and Steven Henikoff created a new labeling technique (Nat. Biotechnol. 18, 424–428, 2000). The researchers linked a bacterial enzyme called DNA adenine methyltransferase (DAM) to the chromatin proteins. When DAM-tagged chromatin binds to DNA, it methylates DNA in the local vicinity, leaving a chemical “footprint”. In the March issue of Nature Genetics (27, 304–308, 2001), van Steensel and colleagues have combined DAM labeling with the power of DNA microarray technology, mapping the location of all binding sites for three chromatin proteins throughout the fruit fly genome. Van Steensel says that the next step will be to build a comprehensive database of the target loci of all chromatin proteins. “We are also adapting chromatin profiling in mammalian cells ... to study the molecular basis of transcriptional programs during cellular differentiation and in diseases such as cancer.” Chromatin maps could provide a more detailed and reproducible profiling technique than current methods using messenger RNA.