Mating-type switching in the fission yeast Schizosaccharomyces pombe results in the transfer of genetic information from one of the two silent cassettes (mat2P or mat3M) to the transcriptionally active locus (mat1). The switching pattern is programmed by an imprinting event which restricts mat1 gene conversion to only one of the two sister cells, leading to asymmetric cell division. Biochemical analysis indicated that the mat1 locus contains a fragile chromosomal site. Southern hybridization and primer extension experiments showed that the fragility consists of a single-strand break (SSB). The nicked DNA is stable throughout the cell cycle. The features of the nick fulfil all the requirements for the 'epigenetic', site and strand-specific chromosome modification at the mat1 locus, providing strong evidence that an SSB can initiate mitotic and meiotic gene conversion during replication.