Piwi-interacting RNA

Piwi-interacting RNA (piRNA) is a class of small RNA molecules that is expressed uniquely in mammalian testes and forms RNA-protein complexes with Piwi proteins. These piRNA complexes (piRCs) have been linked to transcriptional gene silencing of retrotransposons and other genetic elements in germ line cells, particularly those in spermatogenesis. Purification of these complexes has revealed that these oligonucleotides are approximately 29-30 nucleotides long. They are distinct in size from miRNA and are associated with distinct protein complexes.

It remains unclear how piRNAs are generated, but their biogenesis pathway is distinct from miRNA and siRNA.

RNA silencing
piRNA has a role in RNA silencing via the formation of an RNA-induced silencing complex (RISC). The Piwi proteins are part of a family of proteins called the argonautes, which are active in the testes of mammals and are required for germ-cell and stem-cell development in invertebrates. piRNAs are short stretches of RNAs with a typical length of 26-31 nucleotides, making them a distinct entity from microRNAs (miRNAs) and short interfering RNAs (siRNAs) which are 21-23 nucleotides long. Three Piwi subfamily proteins - MIWI, MIWI2 and MILI - have been found to be essential for spermatogenesis in mice.