This study identifies an alternative pathway for microRNA (miRNA) biogenesis, where certain debranched introns mimic the structural features of pre-miRNAs and enter the miRNA-processing pathway without Drosha-mediated cleavage. These introns, termed 'mirtrons,' are found in Drosophila melanogaster and Caenorhabditis elegans. The authors observed small RNA clusters originating from an annotated 56-nucleotide intron in D. melanogaster, which had properties similar to canonical miRNA:miRNA* duplexes. The sequence and secondary structure of this intron resembled pre-miRNAs, suggesting that it did not arise from the canonical miRNA biogenesis pathway but instead from an alternative pathway involving splicing. The authors identified 14 mirtrons in D. melanogaster and four in C. elegans, including the reclassification of mir-62. These mirtrons have been selectively maintained during evolution, indicating important regulatory functions. The abundance of introns comparable in size to pre-miRNAs suggests a context favorable for the emergence of mirtrons in flies and nematodes. The study also demonstrates that mirtrons can direct gene silencing and that their processing depends on splicing and debranching. Knockdown experiments further support the model that mirtrons enter the miRNA biogenesis pathway after splicing and debranching. The authors propose that mirtrons could have emerged early in the evolution of miRNAs before the advent of Drosha.This study identifies an alternative pathway for microRNA (miRNA) biogenesis, where certain debranched introns mimic the structural features of pre-miRNAs and enter the miRNA-processing pathway without Drosha-mediated cleavage. These introns, termed 'mirtrons,' are found in Drosophila melanogaster and Caenorhabditis elegans. The authors observed small RNA clusters originating from an annotated 56-nucleotide intron in D. melanogaster, which had properties similar to canonical miRNA:miRNA* duplexes. The sequence and secondary structure of this intron resembled pre-miRNAs, suggesting that it did not arise from the canonical miRNA biogenesis pathway but instead from an alternative pathway involving splicing. The authors identified 14 mirtrons in D. melanogaster and four in C. elegans, including the reclassification of mir-62. These mirtrons have been selectively maintained during evolution, indicating important regulatory functions. The abundance of introns comparable in size to pre-miRNAs suggests a context favorable for the emergence of mirtrons in flies and nematodes. The study also demonstrates that mirtrons can direct gene silencing and that their processing depends on splicing and debranching. Knockdown experiments further support the model that mirtrons enter the miRNA biogenesis pathway after splicing and debranching. The authors propose that mirtrons could have emerged early in the evolution of miRNAs before the advent of Drosha.