First introns are the longest introns in most species.

<p>Results shown for all species in GenBank release 164 which have at least 500 CDSs that specify multiple introns. Z-tests were used to determine significance and color denotes level of significance (see legend, N.S. = not significant).</p

Incorrect <i>C. elegans</i> gene annotation determined by inspection of intron lengths.

<p>This gene prediction contained an incorrect in-frame intron sequence in the first exon. Transcript evidence, homology evidence from <i>C. briggsae</i>, and an alternative gene prediction (Twinscan) suggested that the first intron is an annotation error. Image taken from Genome Browser display of WormBase release WS180 (<a href="http://ws180.wormbase.org" target="_blank">http://ws180.wormbase.org</a>).</p

Intron size variation for selected species with different numbers of introns.

<p>Intron lengths are shown for species with CDSs that contain 4, 6, 7 or 9 introns (in <i>D. melanogaster, A. thaliana</i>, <i>C. elegans</i>, and <i>H. sapiens</i> respectively). Bars on graph show standard error of the mean. Numbers of CDSs used for each species are shown.</p

Intron length variation in three model organisms.

<p>Mean intron length is calculated for the first intron in the 5′ UTR (position −1, in blue) and for the first eight introns of the coding sequence (in red) for three named species. Error bars indicate standard error of the mean. Bottom right panel shows the occurrence of a potential IME motif (pictured) in <i>A. thaliana</i> introns. %Motif density is calculated by concatenating together all introns in each category, and then calculating what fraction of the total sequence is occupied by the motif.</p