Table 1. piRNA’s biological features over species.

Feature	Species	References
First Uridine	Fly, Mouse, Human, Rat, Nematode (C. elegans), Zebrafish and Silkworm (Bombyx mori)	[17–19]
Tenth Adenine	Human, Fly, Mouse, Zebrafish and Silkworm (Bombyx mori)	[17, 18]
Occurrence in clusters	Mammals and Insects	[18]
Binding with transposons	Mammals and Insects	[19]
CpG islands	Mammals	[22, 27]
G-Quadruplex	Human, Mouse, Rat and Macaque	[26]
Transposable elements presence	Mouse and Marmoset	[24, 25]
Promoter A-Myb	Mouse	[23]
Inverted repeats	Mouse	[24]
Distance to centromeres/telomeres	Fly	[7]
Histone methylation	Fly	[21]