Transcription of miRNA genes. Most miRNAs are transcribed by RNA polymerase II. The miRNAs may be autonomous transcription units or they may be located in other transcripts, including locations within introns and other untranslated regions. Of the polymerase II transcribed miRNAs, about 60% are located in introns of protein coding genes, 12 % are in introns of non-coding RNAs, 18% are in exons of non-coding RNAs, and 10% uncertain.
A second class of miRNA genes are associated with Alu and other repetitive elements and are cotranscribed with these elements by RNA polymerase III. There are currently only a few proven examples of polymerase III transcribed miRNAs.