7C6B:
reference: siRNA potency enhancement via chemical modifications of nucleotide bases at the 5'-end of the siRNA guide strand., Shinohara F, Oashi T, Harumoto T, Nishikawa T, Takayama Y, Miyagi H, Takahashi Y, Nakajima T, Sawada T, Koda Y, Makino A, Sato A, Hamaguch K, Suzuki M, Yamamoto J, Tomari Y, Saito JI, RNA. 2020 Nov 11. pii: rna.073783.119. doi: 10.1261/rna.073783.119. PMID: 33177188

This OCA Structure page
uses Jmol, developed by the Jmol Development Team (documentation).