Whole-genome mapping of Pol II binding in Drosophila S2 cells. (a) Top, percentage of input DNA obtained by ChIP (n = 3; error bars, s.d.) versus chromosome position (in kilobase units), which represents the center point between primers used for quantitative PCR. Bottom, fold enrichment over genomic DNA observed by ChIP-chip (n = 2; error shows range, which is too small to be seen at some data points) versus chromosome position of probes. Relative probe intensities from Pol II ChIP-chip are shown in grayscale (black, higher intensity; white, lower intensity); below, arrows within genes denote the direction of transcription. (b) Flowchart describing the strategy used to determine and validate global Pol II promoter occupancy. (c) A histogram showing the position of the maximally bound probe within each bound gene (n = 5,403) with respect to the transcription start site (TSS).