Genetic and molecular studies have indicated that an epigenetic imprint at  mat1 , the sexual locus of fission yeast, initiates mating type switching. The polar DNA replication of  mat1  generates an imprint on the Watson strand. The process by which the imprint is formed and maintained through the cell cycle remains unclear. To understand better the mechanism of imprint formation and stability, we characterized the recruitment of early players of mating type switching at the  mat1  region. We found that the switch activating protein 1 (Sap1) is preferentially recruited inside the  mat1M  allele on a sequence ( SS13 ) that enhances the imprint. The lysine specific demethylases, Lsd1/2, that control the replication fork pause at  MPS1  and the formation of the imprint are specifically drafted inside of  mat1 , regardless of the allele. The CENP-B homolog, Abp1, is highly enriched next to  mat1  but it is not required in the process. Additionally, we established the computational signature of the imprint. Using this signature, we show that both sides of the imprinted molecule are bound by Lsd1/2 and Sap1, suggesting a nucleoprotein protective structure defined as imprintosome.