My dissertation uses Bakers Yeast, Saccharomyces cerevisiae, to study a double strand break repair pathway called Break- Induced Replication (BIR). This pathway is unusual because it is highly mutagenic and causes genomic rearrangements that can lead to cancer. My work demonstrates that BIR progresses by an unusual replication intermediate leading to the accumulation of single-stranded DNA. Analysis of these structures and the proteins involved provide us a better insight into the mechanism of BIR-associated mutagenesis.