Any of a group of small RNA molecules, typically 24–31 nucleotides long, that interact with piwi domain-containing proteins (named after the Drosophila mutant called P element-induced wimpy testis) or their homologues to form piRNA-induced silencing complexes. These silence transposons in the genome of germline and somatic cells, and are vital for normal formation of sperm and eggs in animals. They also take part in regulating gene expression in various tissues. At least 20 000 piRNAs are expressed in the human genome.