The first intron among all introns within a gene has particularly been a research focus. The first intron is the longest among all other downstream introns within a gene in most species including plants and animals [38]. Additionally, certain transcription factor binding motifs are enriched in first introns [61]. Different parts of genes have different average sizes of introns, e.g., the size of introns in the 5' UTRs are twice as large as introns in coding regions [62]. In Drosophila, long introns evolve more slowly than shorter ones and first introns are the longest compared to other introns [3763]. In Tetrahymena, the introns located closer to the 5' end of genes are more conserved than downstream introns. Our team also proved in a previous study that first introns are the longest and the most conserved [39] compared to other downstream 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
