piRNAs represent the largest class of small non-coding RNA molecules expressed in animal cells, deriving from a large variety of sources, including repetitive DNA and transposons. However, the biogenesis of piRNAs is also the least well understood. piRNAs appear to act both at the post-transcriptional and chromatin levels.  They are distinct from miRNA due to at least an increase in terms of size and complexity. Repeat associated small interfering RNA (rasiRNAs) are considered to be a subspecies of piRNA.