Piwi-interacting RNA (
piRNA) is the largest class of small non-coding
RNA molecules expressed in animal cells. piRNAs form RNA-protein complexes through interactions with
piwi proteins. These piRNA complexes have been linked to both epigenetic and post-transcriptional gene silencing of
retrotransposons and other genetic elements in
germ line cells, particularly those in
spermatogenesis. They are distinct from
microRNA (miRNA) in size (26–31 nt rather than 21–24 nt), lack of sequence conservation, and increased complexity.