 <unk> expressed miRNAs , including both <unk> and intergenic miRNAs , are most important in translational repression and in the regulation of development , especially on the timing of morphogenesis and the maintenance of undifferentiated or incompletely differentiated cell types such as stem cells . The role of endogenously expressed miRNA in downregulating gene expression was first described in C. elegans in 1993 . In plants this function was discovered when the " <unk> microRNA " of Arabidopsis was shown to be involved in the regulation of several genes that control plant shape . In plants , the majority of genes regulated by miRNAs are transcription factors ; thus miRNA activity is particularly wide @-@ ranging and regulates entire gene networks during development by modulating the expression of key regulatory genes , including transcription factors as well as F @-@ box proteins . In many organisms , including humans , miRNAs are linked to the formation of tumors and dysregulation of the cell cycle . Here , miRNAs can function as both oncogenes and tumor suppressors . 
