Equal sister chromatid exchange is a major mechanism of double-strand break repair in yeast.

Equal sister chromatid exchange (SCE) has been thought to be an important mechanism of double-strand break (DSB) repair in eukaryotes, but this has never been proven due to the difficulty of distinguishing SCE products from parental molecules. To evaluate the biological relevance of equal SCE in DSB repair and to understand the underlying molecular mechanism, we developed recombination substrates for the analysis of DSB repair by SCE in yeast. In these substrates, most breaks are limited to one chromatid, allowing the intact sister chromatid to serve as the repair template; both equal and unequal SCE can be detected. We show that equal SCE is a major mechanism of DSB repair, is Rad51 dependent, and is stimulated by Rad59 and Mre11. Our work provides a physical analysis of mitotically occurring SCE in vivo and opens new perspectives for the study and understanding of DSB repair in eukaryotes.