The article provides an overview of the nuclear functions of microRNAs (miRNAs) and their role in gene expression control. MiRNAs, small non-coding RNAs, are crucial regulators of gene expression both in the cytoplasm and nucleus. In the cytoplasm, miRNAs act through the RNA-induced silencing complex (RISC) to degrade target mRNAs or repress translation. However, recent studies have revealed that miRNAs also have specific nuclear functions, including transcriptional control and regulation of alternative splicing. The biogenesis of miRNAs involves a multi-step process, starting with the transcription of primary miRNAs (pri-miRNAs) by RNA polymerases II or III, followed by processing into precursor miRNAs (pre-miRNAs) by the Microprocessor complex, and finally maturation into mature miRNAs by Dicer. MiRNAs can shuttle between the nucleus and cytoplasm, with the minimal RISC (AGO2 and miRNA) being the key component for cytoplasmic activity. In the nucleus, miRNAs can influence the stability of non-coding RNAs, such as long non-coding RNAs (lncRNAs), and affect nucleolar functions, including ribosome biogenesis. Additionally, miRNAs have been shown to participate in transcriptional gene activation (TGA) and transcriptional gene silencing (TGS) at the promoter level, often through interactions with chromatin modifiers and transcription factors. The mechanisms of miRNA-mediated TGA and TGS involve the recognition of complementary sequences in the promoter regions of target genes, leading to changes in chromatin structure and transcriptional activity. Overall, the article highlights the versatility and complexity of miRNA functions, emphasizing their roles in both cytoplasmic and nuclear processes, and their potential as biomarkers in various diseases.The article provides an overview of the nuclear functions of microRNAs (miRNAs) and their role in gene expression control. MiRNAs, small non-coding RNAs, are crucial regulators of gene expression both in the cytoplasm and nucleus. In the cytoplasm, miRNAs act through the RNA-induced silencing complex (RISC) to degrade target mRNAs or repress translation. However, recent studies have revealed that miRNAs also have specific nuclear functions, including transcriptional control and regulation of alternative splicing. The biogenesis of miRNAs involves a multi-step process, starting with the transcription of primary miRNAs (pri-miRNAs) by RNA polymerases II or III, followed by processing into precursor miRNAs (pre-miRNAs) by the Microprocessor complex, and finally maturation into mature miRNAs by Dicer. MiRNAs can shuttle between the nucleus and cytoplasm, with the minimal RISC (AGO2 and miRNA) being the key component for cytoplasmic activity. In the nucleus, miRNAs can influence the stability of non-coding RNAs, such as long non-coding RNAs (lncRNAs), and affect nucleolar functions, including ribosome biogenesis. Additionally, miRNAs have been shown to participate in transcriptional gene activation (TGA) and transcriptional gene silencing (TGS) at the promoter level, often through interactions with chromatin modifiers and transcription factors. The mechanisms of miRNA-mediated TGA and TGS involve the recognition of complementary sequences in the promoter regions of target genes, leading to changes in chromatin structure and transcriptional activity. Overall, the article highlights the versatility and complexity of miRNA functions, emphasizing their roles in both cytoplasmic and nuclear processes, and their potential as biomarkers in various diseases.