Despite the critical roles RNA structures play in regulating gene expression, sequencing-based methods for experimentally determining RNA base pairs have remained inaccurate. Here, we describe a multidimensional chemical-mapping method called "mutate-and-map read out through next-generation sequencing" (M2-seq) that takes advantage of sparsely mutated nucleotides to induce structural perturbations at partner nucleotides and then detects these events through dimethyl sulfate (DMS) probing and mutational profiling. In special cases, fortuitous errors introduced during DNA template preparation and RNA transcription are sufficient to give M2-seq helix signatures; these signals were previously overlooked or mistaken for correlated double-DMS events. When mutations are enhanced through error-prone PCR, in vitro M2-seq experimentally resolves 33 of 68 helices in diverse structured RNAs including ribozyme domains, riboswitch aptamers, and viral RNA domains with a single false positive. These inferences do not require energy minimization algorithms and can be made by either direct visual inspection or by a neural-network-inspired algorithm called M2-net. Measurements on the P4-P6 domain of the Tetrahymena group I ribozyme embedded in Xenopus egg extract demonstrate the ability of M2-seq to detect RNA helices in a complex biological environment.RNA structure modeling | chemical mapping | neural network | mutational profiling | Xenopus egg extract I nference of RNA structures using experimental data is a crucial step in understanding RNA's biological functions throughout living organisms. Chemical-mapping methods have the potential to reveal RNA structural features in situ by probing which nucleotides are protected from attack by chemical modifiers. The resulting experimental data can be used to guide secondary-structure modeling by computational algorithms, raising the prospect of transcriptome-wide RNA structure determination (1, 2).Despite these advances, the accuracy of RNA structure inference through chemical mapping and sequencing remains under question (3-8). For example, models of the 9-kb HIV-1 RNA genome have been repeatedly revised with updates to the selective 2′-OH acylation by primer extension (SHAPE) protocol, data processing, and computational assumptions (2, 9-11), and the majority of this RNA's helices remain uncertain. Even for small RNA domains, SHAPE and dimethyl sulfate (DMS) (methylation of N1 and N3 atoms at A and C) have produced misleading secondary structures for ribosomal domains and blind modeling challenges that have been falsified through crystallography or mutagenesis (3,7,12,13). In alternative approaches based on photoactivated cross-linkers, many helix detections appear to be false positives, based on ribosome data in vitro and in vivo (14,15).The confidence and structural accuracy of chemical-mapping methods can be improved by applying perturbations to the RNA sequence before chemical modification. In the mutate-and-map strategy, mapping not just the target RNA sequence but also a compre...