Alternative RNA folding generates miRNAs by sharing one arm of a stem. Examples of alternative RNA folding in precursors of Bdi-miR9483 (a), Bdi-miR2118 (b) and Bdi-miR166 (c). Shown are schematic diagrams of adjacent miRNA precursors that share one arm of a stem. miRNA placement along the precursor is indicated with arrows, the middle arrow representing the shared arm of two miRNA precursors and the abundance of small RNAs matching each position is plotted below. Purple and red lines and filled circles indicate two mature miRNAs corresponding to each precursor and the blue line and filled circle indicates the miRNA*. Predicted secondary structures from each precursor, as well as that from the full sequence, are shown in the right panel. (d) Model for alternative RNA folding generating miRNAs by sharing one arm of a stem. The nascent transcript preferentially processed by DCL activity generates the first miRNAs (purple). After transcription, two possible RNA folding structures are responsible for generating the two mature miRNAs (purple and red). DCL, Dicer-like protein; miRNA, microRNA; TP2M, transcripts per 2 million.