 The branch site, both exons, the catalytically essential regions of DV and J2/3, and ε−ε' are in close proximity before the first step of splicing occurs.In addition to the bulge and AGC triad regions of DV, the J2/3 linker region, the ε−ε' nucleotides and the coordination loop in DI are crucial for the architecture and function of the active-site.The first crystal structure of a group II intron was resolved in 2008 for the "Oceanobacillus iheyensis" group IIC catalytic intron, and was joined by the "Pylaiella littoralis" (P.li.LSUI2) group IIB intron in 2014.