text: Many model organism, such as plants (Arabidopsis thaliana), yeast (Saccharomyces cerevisiae  ), flies (Drosophila melanogaster) and worms (C. elegans), have been used to study small non coding RNA-directed Transcriptional gene silencing. In human cell, RNA-directed transcriptional gene silencing was observed a decade ago when exogenous siRNAs silenced a transgenic elongation factor 1 α promoter driving a Green Fluorescent Protein (GFP) reporter gene.
  The main mechanisms of transcriptional gene silencing (TGS) involving the RNAi machinery include DNA methylation, histone post-translational modifications, and subsequent chromatin remodeling around the target gene into a heterochromatic state.
  SiRNAs can be incorporated into a RNA-induced transcriptional silencing (RITS) complex. An active RITS complex will trigger the formation of heterochromatin around DNA matching the siRNA, effectively silencing the genes in that region of the DNA.