Intron-poor and introns-rich genomes in the descendants of the earliest cellular life forms. Group I introns are ribozymes. Group II introns are self-splicing mobile ribozymes spanning in the proto-organelles from archaea/prokaryota/eubacteria to eukaryota. They may encode proteins for their own folding into secondary or tertiary structures. Spliceosomal nuclear introns presumably derive from group II introns of the mitochondrial/chloroplastic endosymbionts (vide infra). These introns are not present in a-nucleate archaeal or prokaryotic genomes, but are eukaryotic nuclear-genomically encoded pre-mRNA sequences (in the spliceosomes). In the gene, they reside between protein-encoding DNA segments (exons). The spliceosomes catalyze the removal of these introns from the mRNAs. Extranuclear (cytoplasmic) processing of group II intron-containing transcripts undergo transitional repression and nonsense-mediated decay (NMD). Group I introns, or nuclear spliceosomal introns are processed without NMD, or translational repression . Introns vary not only in numbers from genera to genera, but also in size from twenty nt (in the nucleomorphs) to kilobase sizes in some eukaryote metazoans (Homo). The common consensus is 5′GT at one end (“donor” site), and 3′ AG at the other end (“acceptor” site). The branch point is somewhere within the intron’s sequences.