This study identifies the genomic locations and contexts of microRNAs (miRNAs) in human and mouse genomes, revealing that most miRNAs are transcribed within introns or exons of protein-coding or noncoding genes. Of the 232 known mammalian miRNAs, over half are located in introns of protein-coding or noncoding transcripts, while about 10% are found in exons of long nonprotein-coding transcripts. The study also identifies novel miRNA families based on host gene identity and shows that miRNAs are transcribed in parallel with their host transcripts. Two distinct classes of miRNAs are identified: 'exonic' miRNAs located in exons and 'intronic' miRNAs located in introns. These findings suggest that miRNAs may require different biogenesis mechanisms depending on their transcriptional class. The study also highlights the importance of miRNA host genes in regulating miRNA expression across various biological contexts. The results indicate that miRNAs are often associated with complex transcriptional loci and that their expression patterns can be inferred from host gene expression data. The study provides insights into the functional roles of miRNAs and their potential regulatory roles in gene expression. The findings have important implications for understanding the regulation of gene expression and the definition of what constitutes a gene.This study identifies the genomic locations and contexts of microRNAs (miRNAs) in human and mouse genomes, revealing that most miRNAs are transcribed within introns or exons of protein-coding or noncoding genes. Of the 232 known mammalian miRNAs, over half are located in introns of protein-coding or noncoding transcripts, while about 10% are found in exons of long nonprotein-coding transcripts. The study also identifies novel miRNA families based on host gene identity and shows that miRNAs are transcribed in parallel with their host transcripts. Two distinct classes of miRNAs are identified: 'exonic' miRNAs located in exons and 'intronic' miRNAs located in introns. These findings suggest that miRNAs may require different biogenesis mechanisms depending on their transcriptional class. The study also highlights the importance of miRNA host genes in regulating miRNA expression across various biological contexts. The results indicate that miRNAs are often associated with complex transcriptional loci and that their expression patterns can be inferred from host gene expression data. The study provides insights into the functional roles of miRNAs and their potential regulatory roles in gene expression. The findings have important implications for understanding the regulation of gene expression and the definition of what constitutes a gene.