Splicing of all intron-containing RNA molecules is superficially similar, as described above. However, different types of introns were identified through the examination of intron structure by DNA sequence analysis, together with genetic and biochemical analysis of RNA splicing reactions. At least four distinct classes of introns have been identified:
*Introns in nuclear protein-coding genes that are removed by spliceosomes (spliceosomal introns)
* Introns in nuclear and archaeal transfer RNA genes that are removed by proteins (tRNA introns)
* Self-splicing group I introns that are removed by RNA catalysis
* Self-splicing group II introns that are removed by RNA catalysis
Group III introns are proposed to be a fifth family, but little is known about the biochemical apparatus that mediates their splicing. They appear to be related to group II introns, and possibly to spliceosomal introns.