miRBase is the primary online repository for microRNA sequences and annotations, currently containing over 15,000 microRNA gene loci and 17,000 distinct mature microRNA sequences from 142 species. The repository has seen a significant increase in the number of microRNAs due to deep-sequencing technologies, which have facilitated the discovery of novel microRNAs. miRBase has integrated deep-sequencing data by mapping reads to microRNA loci and providing web interfaces to view these mappings. Users can filter reads by experiment and tissue-specific expression, and use read counts as a proxy for relative expression levels. The integration of deep-sequencing data helps in distinguishing microRNAs from other RNA species and fragments, and aids in refining annotations. Future developments include improving community contribution and access to microRNA data, expanding the database to include microRNA candidates and predictions, and developing a microRNA target aggregation service.miRBase is the primary online repository for microRNA sequences and annotations, currently containing over 15,000 microRNA gene loci and 17,000 distinct mature microRNA sequences from 142 species. The repository has seen a significant increase in the number of microRNAs due to deep-sequencing technologies, which have facilitated the discovery of novel microRNAs. miRBase has integrated deep-sequencing data by mapping reads to microRNA loci and providing web interfaces to view these mappings. Users can filter reads by experiment and tissue-specific expression, and use read counts as a proxy for relative expression levels. The integration of deep-sequencing data helps in distinguishing microRNAs from other RNA species and fragments, and aids in refining annotations. Future developments include improving community contribution and access to microRNA data, expanding the database to include microRNA candidates and predictions, and developing a microRNA target aggregation service.