Until recently, scientists assumed that most of the remaining DNA was not transcribed, thinking that since it did not specify proteins or the few known types of RNA, such DNA did not contain meaningful genetic information. a
Since 1993, a number of studies have uncovered microRNAs (miRNAs)––small, single-stranded RNA molecules capable of binding to complementary sequences in mRNA molecules. d
There are ca.1,500 genes for miRNAs in the human genome, and biologists estimate that expression of at least one-half of all human genes may be regulated by miRNAs, a remarkable number given that the existence of miRNAs was unknown until the early 1990s. f