bridged nucleicacids (2',4'-BNA) or lockednucleicacids (LNA). Both the 2'-O,4'-C-ethylene- and propylene-linkage within these nucleosides restrict the sugar puckering to the N-conformation of RNA as do 2',4'-BNA/LNA. Furthermore, ethylene-bridged nucleicacids (ENA) having 2'-O,4'-C-ethylene nucleosides had considerably increased the affinity to complementary RNA, and were as high as that of 2',4'-BNA/LNA