Sequence and structure of siRNA guide strands. Blue: 2′-O-Me, Green: 2′-F, Orange: 2′-O-MOE, VP-T = 2′-O-MOE-thymidine-(E)-5′-vinylphosphonate; P: phosphate, s: phosphorothioate; all backbone linkages as phosphodiesters except those indicated with s; X1: abasic; X2: UNA.