 Although most of its genome can be characterized as euchromatin, "S. cerevisiae" has regions of DNA that are transcribed very poorly.These loci are the so-called silent mating type loci (HML and HMR), the rDNA (encoding ribosomal RNA), and the sub-telomeric regions.Fission yeast ("Schizosaccharomyces pombe") uses another mechanism for heterochromatin formation at its centromeres.