Human chromosomes often undergo breakage due to agents that damage the DNA. It is critical to repair such breaks, to maintain genome integrity and to prevent mutations that can give rise to cancer. All organisms have the capacity to repair such breaks by a process called homologous recombination, which restores the continuity of the genome without introducing mutations. A non-homologous recombination process, called end-joining, often introduces mutations and thus is only used by a cell when homologous recombination is not possible. Maria Jasin pioneered genetic and physical assays for recombination in human cells and she was the first scientist to directly demonstrate the importance of both homologous recombination and non-homologous end-joining for repair of chromosomal breaks. Her discovery has important implications for both normal cellular function, embryonic development, fertility, and for the etiology of diseases such as cancer. In the course of this work, Dr Jasin demonstrated that breaks in chromosomes greatly increase the frequency of recombination at the site of the break. This important discovery laid the groundwork for efficient modification of mammalian genomes by site-specific nucleases, an approach that is currently being widely exploited for gene therapy and basic research.