introns. Furthermore, we showed that active histone marks, such as H3K4me1, and H3K4me3, are significantly enriched in the first introns, and the size of the first intron of a gene becomes bigger as the number of exons that gene carries increases. Additionally, we showed in the same paper that the proportions of regulatory histone marks are positively associated with the levels of gene expressions in 12 normal human tissues including kidney, heart, liver, and ovary [39].
