Sir3 gene silencer acting on DNA, molecular model. Sir3 (light blue) is acting on a circular strand of DNA (deoxyribonucleic acid, pink). Sir3 is an example of a silent information regulator (Sir) found in yeast such as Saccharomyces cerevisiae. Here, the DNA is wrapped around a histone core (orange-green), forming a structure called a nucleosome. The region of the Sir3 gene silencer shown here is the BAH (bromo-associated homology) domain.