Group II introns are a large class of self-catalytic ribozymes as well as mobile genetic element found within the genes of all three domains of life. Ribozyme activity (e.g., Self-splicing) can occur under high-salt conditions in vitro. However, assistance from proteins is required for in vivo splicing . In contrast to group I introns, intron excision occurs in the absence of GTP and involves the formation of a lariat, with an A-residue branchpoint strongly resembling that found in lariats formed during splicing of nuclear pre-mRNA. It is hypothesized that pre-mRNA splicing (see spliceosome) may have evolved from group II introns, due to the similar catalytic mechanism as well as the structural similarity of the Domain V substructure to the U6/U2 extended snRNA. Finally, their ability to site-specifically mobilize to new DNA sites has been exploited as a tool for biotechnology.