Secondary structures of the 5′ BC1 domain and ID1–ID4 elements. Rat consensus sequences (Kim et al., 1994) are shown. Secondary structure representation is based on previous evidence (Rozhdestvensky et al., 2001; Muslimov et al., 2006, 2011). Base pairs are denoted as follows: =, GC standard WC; –, AU standard WC, ·, GU wobble WC; •, noncanonical (non-WC). GA motif structures are bracketed by vertical bars. Noncanonical purine•purine base pairs (A•A, A•G, G•A, and G•G) are shown in red. GA motif WC base pairs clamping the noncanonical GA motif core are shown in blue. Differences between ID elements and the 5′ BC1 domain are indicated by arrows. The 5′ BC1 domain and ID1 are identical; 5′ BC1 nucleotide numbering is shown.