Kalyna and colleagues characterized protein-coding exonic sequences that can undergo alternative splicing, which they termed exonic introns (exitrons; previously termed cryptic introns). They defined >1,000 exitrons in >3% of Arabidopsis thaliana coding genes, including in many genes annotated as intron-less, and found that they possess sequence characteristics different from those of exons and introns. Expression analysis of ten genes with exitrons that preserve the reading frame upon splicing revealed tissue-, stress- and development-dependent exitron splicing, which was affected by the expression of several alternative splicing factors. Exitron abundance and characteristics are conserved between the A. thaliana and human genomes; such characteristics include tissue-dependent alternative splicing and enrichment in protein disordered regions, short linear motifs, and phosphorylation and ubiquitylation sites. The majority of exitrons seem to have originated from exons, but some are probably derivatives of intron degeneration.