We thank James Haber (Brandeis University) for use of the BIR assay strain JRL346, Marco Foiani (University of Milan) for providing the α-Rad53 antibody, and Greg Ira (Baylor College of Medicine) for thoughtful input and use of reagents for this project. We are grateful to Bertuch lab members, especially to Christopher Williams for technical assistance and Nimrat Chatterjee and Ann Sukumar for thoughtful comments and editing of the manuscript. The project described was supported by the National Institutes of Health, National Institute of General Medical Sciences [T32GM008231 supporting E.J.P. and GM077509 to A.A.B.]. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute of General Medical Sciences or the National Institutes of Health.