MicroRNAs (miRNA) are non-coding RNA around 22 nucleotides in length. They are responsible for gene expression and preventing translation of mRNA. miRNAs are highly conserved in evolutionary terms .
miRNA are capable of regulating many RNA and in combination with each other.
One of the first families of miRNA, Let-7, was discovered in Caenorhabditis elegans. Let-7 has since been discovered in humans and is controlled at post-transcriptional level. Misregulation of Let-7 can lead to uncontrolled cell growth and cancer .