Table 1.

Differences between siRNA and miRNA

	siRNA	miRNA
Occurrence	Occurs naturally in plants and animals. It is currently unknown whether or not they occur naturally in mammals.	Occurs naturally in plants and animals.
Mean length	Approx. 21–22 nt	Approx. 19–25 nt
Complementarity to target mRNA	100% perfect match; therefore, siRNAs knock down specific genes, with minor off-target exceptions.	Not exact; therefore, a single miRNA may target up to hundreds of mRNAs.
Biogenesis	Regulate the same genes that express them.	Expressed by genes whose purpose is to make miRNAs, but they regulate genes (mRNAs) other than the ones that expressed them.
Action	Cleave mRNA.	Inhibit or replace translation of mRNA.
Function	Act as gene silencing guardians in plants and animals that do not have antibody- or cell-mediated immunity.	Regulators (inhibitors) of genes (mRNAs)