The reversible state of proliferative arrest known as "cellular quiescence" plays an important role in tissue homeostasis and stem cell biology. By analyzing the expression of miRNAs and miRNAprocessing factors during quiescence in primary human fibroblasts, we identified a group of miRNAs that are induced during quiescence despite markedly reduced expression of Exportin-5, a protein required for canonical miRNA biogenesis. The biogenesis of these quiescence-induced miRNAs is independent of Exportin-5 and depends instead on Exportin-1. Moreover, these quiescence-induced primary miRNAs (pri-miRNAs) are modified with a 2,2,7-trimethylguanosine (TMG)-cap, which is known to bind Exportin-1, and knockdown of Exportin-1 or trimethylguanosine synthase 1, responsible for (TMG)-capping, inhibits their biogenesis. Surprisingly, in quiescent cells Exportin-1-dependent pri-miR-34a is present in the cytoplasm together with a small isoform of Drosha, implying the existence of a different miRNA processing pathway in these cells. Our findings suggest that during quiescence the canonical miRNA biogenesis pathway is down-regulated and specific miRNAs are generated by an alternative pathway to regulate genes involved in cellular growth arrest.M ost metazoan cells enter a reversible cell-cycle arrest known as "cellular quiescence" when they are exposed to antimitogenic signals or an environment unfavorable for proliferation (1, 2). In mammalian cells, quiescence (also known as "G 0 arrest") is characterized by reduced DNA replication, altered metabolism, increased autophagy, and increased expression of cyclin-dependent kinase inhibitors such as p27 Kip1 (3,4). In vitro, quiescence can be induced in primary cells by serum starvation, contact inhibition, and loss of adhesion to a substrate (5). Quiescence is involved in important cellular processes, including the balance between differentiation and self-renewal in different types of stem cells, and dysregulation of quiescence could favor carcinogenesis. However, the molecular mechanisms that regulate quiescence are poorly understood (6). miRNAs are small, noncoding RNAs ∼22-nt long that regulate the expression of protein-coding genes by base-pairing with the 3′ UTR of mRNAs, repressing the translation and/or inducing the degradation of the target mRNA (7,8). In canonical miRNA biogenesis in mammalian cells, miRNAs are transcribed by RNA polymerase II to produce 7-methylguanosine (m 7 G)-capped primary miRNAs (pri-miRNAs) containing one or more bulged hairpin structures that are recognized by the nuclear microprocessor, which consists of the RNase III enzyme Drosha and the dsRNA-binding protein DGCR8 (DiGeorge syndrome critical region gene 8) (9-12). Specific cleavage of the pri-miRNA by the nuclear microprocessor generates an ∼70-nt stem-loop structure known as the "precursor miRNA" (pre-miRNA), which is recognized and transported to the cytoplasm by 10,11,13). Subsequently, pre-miRNA is cleaved by the cytoplasmic RNase III enzyme Dicer (14-17), generating a double-stranded mature mi...