Proportions of regulatory chromatin marks in intron ordinal groups. All the signals are derived from GM12878 cell line. Using the peak values for each signal, box plot analysis is performed for the proportions of the chromatin marks sites in introns from each ordinal group are estimated. Results of the same analyses in the two other cell lines are presented in Additional file 1: Figure S2A, B. The proportions of the peak signals of each chromatin regulatory marks in first introns are represented by darker gray colors than those in the other downstream introns. As shown, the proportions of the regulatory chromatin marks are found to be the highest in first introns compared to the other downstream introns.