MicroRNAs (miRNAs) constitute a large family of noncoding RNAs that function as 
guide molecules in diverse gene silencing pathways. Current efforts are focused 
on the regulatory function of miRNAs, while little is known about how these 
unusual genes themselves are regulated. Here we present the first direct 
evidence that miRNA genes are transcribed by RNA polymerase II (pol II). The 
primary miRNA transcripts (pri-miRNAs) contain cap structures as well as poly(A) 
tails, which are the unique properties of class II gene transcripts. The 
treatment of human cells with alpha-amanitin decreased the level of pri-miRNAs 
at a concentration that selectively inhibits pol II activity. Furthermore, 
chromatin immunoprecipitation analyses show that pol II is physically associated 
with a miRNA promoter. We also describe, for the first time, the detailed 
structure of a miRNA gene by determining the promoter and the terminator of 
mir-23a approximately 27a approximately 24-2. These data indicate that pol II is 
the main, if not the only, RNA polymerase for miRNA gene transcription. Our 
study offers a basis for understanding the structure and regulation of miRNA 
genes.