A microRNA array reveals extensive regulation of microRNAs during brain development. Researchers developed an oligonucleotide array to analyze miRNA expression in brain tissues and cells. The array, designed for 44 mature miRNAs, showed precise regulation of miRNA expression during brain development. About 20% of the probed miRNAs changed significantly during normal brain development, with miR-9 and miR-131 dysregulated in presenilin-1 null mice. These miRNAs were validated by Northern blots, and bioinformatic analysis suggested potential mRNA targets. The arrays also revealed miRNAs associated with translating polyribosomes in primary neurons, likely modulating translation. Oligonucleotide arrays provide a new tool for studying miRNA expression in various biological and pathobiological settings. Clustering coexpressed miRNAs will help understand their regulation, functions, and mRNA targets. MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression by binding to target mRNAs. They are involved in various biological processes, including development, and are conserved across species. The study shows that miRNA expression is tightly regulated during brain development, with specific miRNAs like miR-9 and miR-131 dysregulated in mice lacking presenilin-1, which exhibit severe brain developmental defects. The miRNAs were found to be associated with translating polyribosomes, suggesting a role in translation regulation. The study also identified potential mRNA targets for developmentally regulated miRNAs, such as calcineurin Aβ and Id2. The findings highlight the importance of miRNAs in brain development and their potential as therapeutic targets in neurological disorders. The study used an oligonucleotide array to analyze miRNA expression, providing a new tool for studying miRNA function in various biological contexts. The results suggest that miRNAs play a crucial role in regulating brain development and may be involved in diseases such as spinal muscular atrophy. The study also highlights the need for further research into miRNA functions and their potential applications in medicine.A microRNA array reveals extensive regulation of microRNAs during brain development. Researchers developed an oligonucleotide array to analyze miRNA expression in brain tissues and cells. The array, designed for 44 mature miRNAs, showed precise regulation of miRNA expression during brain development. About 20% of the probed miRNAs changed significantly during normal brain development, with miR-9 and miR-131 dysregulated in presenilin-1 null mice. These miRNAs were validated by Northern blots, and bioinformatic analysis suggested potential mRNA targets. The arrays also revealed miRNAs associated with translating polyribosomes in primary neurons, likely modulating translation. Oligonucleotide arrays provide a new tool for studying miRNA expression in various biological and pathobiological settings. Clustering coexpressed miRNAs will help understand their regulation, functions, and mRNA targets. MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression by binding to target mRNAs. They are involved in various biological processes, including development, and are conserved across species. The study shows that miRNA expression is tightly regulated during brain development, with specific miRNAs like miR-9 and miR-131 dysregulated in mice lacking presenilin-1, which exhibit severe brain developmental defects. The miRNAs were found to be associated with translating polyribosomes, suggesting a role in translation regulation. The study also identified potential mRNA targets for developmentally regulated miRNAs, such as calcineurin Aβ and Id2. The findings highlight the importance of miRNAs in brain development and their potential as therapeutic targets in neurological disorders. The study used an oligonucleotide array to analyze miRNA expression, providing a new tool for studying miRNA function in various biological contexts. The results suggest that miRNAs play a crucial role in regulating brain development and may be involved in diseases such as spinal muscular atrophy. The study also highlights the need for further research into miRNA functions and their potential applications in medicine.