 Since the discovery of the psbF twintron, several categories of twintrons have been characterized.A twintron can be simple (external intron interrupted by 1 internal intron), or complex (external intron interrupted by multiple internal introns).Most probably, the internal and external introns comprising the twintron element are from the same category; group I internal to group I, group II internal to group II, and group III internal to group III.