The study of transcriptional processes in higher eukaryotes has been limited by the scarce availability of in vivo assays. This shortage of technical approaches has become more important in light of the emerging notion that the natural chromatin context affects the outcome of transcriptional activation in vivo (1). Chromatin can exert a regulatory effect on transcription by modulating the access of activators to DNA (1). Different posttranslational modifications of specific residues in the N-terminal tails of the nucleosomal histones have been characterized as a signal code that is linked to the active or inactive transcriptional status of promoters (2). These modifications, including acetylation, methylation, phosphorylation, and ubiquitination, are thought to result from the targeted binding to promoters of transcriptional coactivator or core-pressor complexes containing such enzymatic activities. These complexes do not generally possess DNA-binding activity, but are recruited to promoters by their interaction with sequence-specific transcription factors (3). The resulting modifications of the histone tails, in turn, modulate the access of further regulatory complexes to the promoter (4). This subtle regulatory interplay can easily be missed when transcriptional activation or transcription factor binding to target promoters is studied in vitro using naked DNA templates.