From the discovery of DNA as carrier of the genetic information in the 1940’s until the publication of the human genome in 2001 life science knowledge has increased enormously. However, unlike it was assumed before, not the nucleotide sequence length or gene number alone, but the precise control of the gene expression is crucial for the development of higher organisms. To solve the questions concerning this regulation procedure is the task of the post-genomic era addressed by the field of epigenetics. In the work presented here, the regulation of DNA replication and silencing of the silent mating type locus HML in the baker’s yeast Saccharomyces cerevisiae were analysed. Although the initiation of DNA replication and gene silencing by heterochromatin formation are two distinct processes, both share key components.