1 DNA Recombination and Repair Laboratory, The Francis Crick Institute, London NW1 1AT, United Kingdom; stephen.west@crick.ac.uk sckowalczykowski@ucdavis.edu.
2 Department of Microbiology and Molecular Genetics, University of California, Davis, CA 95616; stephen.west@crick.ac.uk sckowalczykowski@ucdavis.edu.
3 Department of Molecular and Cellular Biology, University of California, Davis, CA 95616.