An intron (or intervening sequence) is a segment of RNA excised from a gene transcript, with concomitant ligation of flanking segments called exons. Introns of some type are found in every kingdom of cellular life, and also in viruses, bacteriophages and plasmids. The spliceosomal introns are the most familiar class of introns, and are found primarily in protein-coding genes in the eukaryotic nucleus. For group I and group II self-splicing introns, the introns RNA plays a primary role in the biochemistry of splicing, in some cases being sufficient for splicing in vitro. Introns apparently have no general role in gene expression, through in specific cases, introns and splicing figure importantly in developmental gene expression.