TRANSCRIPTIONAL silencing of the HM mating-type loci in the yeast Saccharomyces cerevisiae is caused by the localized formation of an altered chromatin structure, analogous to heterochromatin in higher eukaryotes. Silencing depends on cis-acting sequences, termed silencers, as well as several trans-acting factors, including histones H4 and H3, proteins RAP1 and ABF1, and the four SIR proteins (SIR1-4)(1). Each of the four HM silencers contains an autonomously replicating sequence (ARS) to which the origin replication complex (ORC)(2) binds. This six-protein complex is required for initiation of DNA replication, as well as for silencing(3-7). Efficient establishment of the silenced state requires both passage through the S phase of the cell cycle(8) and SIR1 protein(9,10). Previous experiments suggested that SIR1 might be localized to the silencers by binding to ORC and/or RAP1 (ref. 11). Here we report that SIR1 can bind directly to ORC1, the largest of the ORC subunits, and that targeting of SIR1 to ORC1 at a silencer is sufficient to establish a silenced state.