MicroRNAs (miRNAs) appear to regulate the expression of more than 60% of protein coding genes of the human genome.   If an miRNA is abundant it can behave as a "switch", turning some genes on or off. However, altered expression of many miRNAs only leads to a modest 1.5- to 4-fold change in protein expression of their target genes.   Individual miRNAs often repress several hundred target genes.   Repression usually occurs either through translational silencing of the mRNA or through degradation of the mRNA, via complementary binding, mostly to specific sequences in the 3 untranslated region of the target genes mRNA.   The mechanism of translational silencing or degradation of mRNA is implemented through the RNA-induced silencing complex (RISC).