(i) MicroRNAs (miRNAs) are transcribed by RNA polymerase II from intragenic or intergenic regions of the genome. (ii) This transcript, the primary miRNA (pri-miRNA), can range 100–1000 nucleotides in length and shares many properties of mRNA transcripts, including a 7-methylguanasine (m7G) 5′-cap and poly-A tail. Within pri-miRNA, ~70bp stem-loops are recognized and cleaved by the complex of DGCR8 (DiGeorge syndrome critical region gene 8) and the RNAse III endonuclease enzyme, Drosha, to produce the precursor miRNA (pre-miRNA). (iii) Exportin 5 then transports the pre-miRNA from the nucleus to the cytoplasm, where (iv) the RNAse III endonuclease, Dicer, cleaves the stem away from the loop of the pre-miRNA to produce the (v) mature miRNA duplex. (vi) One strand of this ~21nt long duplex is incorporated into the RNA induced silencing complex (RISC) to regulate the translation/degradation of target mRNAs. (b) Whereas exogenous addition of dsRNA (siRNA) to the cell is a common experimental manipulation, recent evidence suggests that endogenous forms of siRNA also exist in animals. (i) Endogenous siRNA forms when long complementary strands of RNA bind to form dsRNA. The individual strands of these dsRNA are products of pseudogenes, transposable elements, or protein-coding genes, and are the result of complementary transcriptional products from cis or trans loci (same or different chromosomes, respectively) or the result of inverted repeat sequences that have folded to form a hairpin structure [9, 10]. (ii) Within the cytoplasm, dsRNA is cleaved into multiple ~21nt fragments by Dicer. (iii) One strand from these duplexes is incorporated into a (siRISC) to bind target mRNA and facilitate siRNA mediated transcript degradation. For a more detailed explanation of miRNA and siRNA biogenesis, see the following [58–60].