For many years, only a small fraction of the human genome was believed to regulate cell function and development. This protein-coding portion composed only 1% to 2% of 3 billion human DNA base pairs—the remaining sequence was classified as junk DNA. Subsequent research has revealed that most of the genome is transcribed into a broad array of noncoding RNAs, ranging in size from microRNA (20–23 nucleotides) to long noncoding RNA (lncRNA, more than 200 nucleotides). These noncoding RNA classes have been shown to use diverse molecular mechanisms to control gene expression and organ system development.