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Predicted isoforms of canonical GNRA tetraloops presented in L9 and L9.2 of some IE introns. Standard base-pairing resulted in the loops (left). If non-canonical A–G base pair and bulges are allowed, the loops in (A) and (C) become GNRA tetraloops, while that in (B) turns to GAUAA. See Figure 2 and Supplementary Figure S1 for the receptor structure of the L9 GNRA loop, which is located in P5.
