MicroRNAs (miRNAs) are small non-coding RNA molecules that regulate gene expression by binding to target mRNAs, either by cleavage or translational repression. This study identifies 2,000 human genes with conserved miRNA target sites in mammals and 250 genes conserved between mammals and fish. The prediction algorithm uses sequence complementarity and interspecies conservation to identify target sites. Experimental validation supports the method, showing enrichment of predicted targets in mRNAs associated with the fragile X mental retardation protein (FMRP), suggesting miRNAs act as sequence-specific adaptors in ribonuclear particle interactions. Overrepresented target groups include transcription factors, miRNA machinery components, and proteins involved in translational regulation. The study also highlights the role of miRNAs in regulating 10% or more of human genes, with miRNA genes accounting for about 1% of all human genes. The miRanda software is provided for target prediction. The study also explores miRNA functions in various organisms, including their roles in development, apoptosis, and disease. The study validates miRNA targets using experimental data and shows that miRNAs regulate a wide range of biological processes, including transcription, translation, and protein stability. The results suggest that miRNAs are involved in complex regulatory networks, with some miRNAs targeting multiple genes and others being targeted by multiple miRNAs. The study also identifies miRNA targets in cancer-related genes, showing that miRNAs may play a role in cancer development and progression. The study provides a comprehensive analysis of miRNA target sites in human, mouse, and rat genomes, highlighting the importance of miRNAs in gene regulation and disease.MicroRNAs (miRNAs) are small non-coding RNA molecules that regulate gene expression by binding to target mRNAs, either by cleavage or translational repression. This study identifies 2,000 human genes with conserved miRNA target sites in mammals and 250 genes conserved between mammals and fish. The prediction algorithm uses sequence complementarity and interspecies conservation to identify target sites. Experimental validation supports the method, showing enrichment of predicted targets in mRNAs associated with the fragile X mental retardation protein (FMRP), suggesting miRNAs act as sequence-specific adaptors in ribonuclear particle interactions. Overrepresented target groups include transcription factors, miRNA machinery components, and proteins involved in translational regulation. The study also highlights the role of miRNAs in regulating 10% or more of human genes, with miRNA genes accounting for about 1% of all human genes. The miRanda software is provided for target prediction. The study also explores miRNA functions in various organisms, including their roles in development, apoptosis, and disease. The study validates miRNA targets using experimental data and shows that miRNAs regulate a wide range of biological processes, including transcription, translation, and protein stability. The results suggest that miRNAs are involved in complex regulatory networks, with some miRNAs targeting multiple genes and others being targeted by multiple miRNAs. The study also identifies miRNA targets in cancer-related genes, showing that miRNAs may play a role in cancer development and progression. The study provides a comprehensive analysis of miRNA target sites in human, mouse, and rat genomes, highlighting the importance of miRNAs in gene regulation and disease.