Table 1.

Proposed mechanisms for licensing of RNA silencing pathways

RNA silencing pathway	Organism	Proposed licensing mechanism	Ref.
Endogenous siRNA	H. sapiens	Formation of dsRNA substrates for Dicer	[12, 17–20, 23]
	M. musculus
	D. melanogaster
	C. elegans
	Z. mays
	S. castellii
Quelling	N. crassa	Unusual DNA repair structure at tandem repeat locus	[32]
Meiotic silencing of unpaired DNA	N. crassa	Unpaired DNA in meiosis I	[33, 38]
	C. elegans
Endogenous siRNA	C. neoformans	Stalled splicing of target transcript	[40]
Post-transcriptional gene silencing	A. thaliana	Improper target transcript 5' or 3' end formation	[52–56]
piRNA	D. melanogaster	Privileged chromosomal loci for piRNA production (e.g., specialized chromatin)	[62, 74]
	C. elegans	Absence of self-protective signals in target (e.g., CSR-1 targeting?)	[82, 83]