Schematic representation of the distribution of the key regulators of dosage compensation on a target gene in Drosophila. The gene is depicted as a gray bar at the top of the figure, with the arrow representing the transcription start site. The figure is based on genome-wide binding studies of MOF, MBD-R2 and MSL1. The upper panel shows that MBD-R2 is enriched at promoters (5') on all chromosomes in both sexes, underscoring its function as a general transcriptional facilitator. MOF co-localizes with the promoter peak of MBD-R2 on all chromosomes except for the male X chromosome, where it is more enriched towards the 3' end of the target gene as a result of its association with the dosage-compensation complex (bottom panel). The MSL1 profile serves as a marker for the presence of the dosage-compensation complex . For details see text.