Pirna is a town in the Free State of Saxony, Germany, capital of the administrative district Sächsische Schweiz-Osterzgebirge. The town's population is over 37,000. Pirna is located near Dresden and is an important district town as well as a Große Kreisstadt. It is also known for the gassing of about 15,000 disabled people in Sonnenstein Castle between June 1940 and August 1941, killings which ceased after pressure was exerted on the authorities by the local population. Today, a small plaque at the base of Sonnenstein Castle and the Sonnenstein Memorial remembers the dead.

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.