Welcome to the Walter lab home page at Harvard Medical School. We study how vertebrate cells faithfully duplicate their genomes in S phase. We perform most of our experiments in egg extracts from the African clawed toad, Xenopus laevis. These extracts are remarkable in that they recapitulate key processes underlying genome propagation, including DNA replication, the ATR and ATM checkpoints, chromosome cohesion, ubiquitin-mediated proteolysis, many DNA repair pathways, and the segregation of chromosomes in mitosis. We are using egg extracts to understand the molecular machines that carry out DNA replication and how these machines are regulated by ubiquitin-mediated proteolysis to limit genome duplication to exactly one round per cell cycle. We also study DNA inter-strand cross-link repair in S phase and how this process goes awry in the human genetic disorder, Fanconi anemia. Recently, we have developed a new imaging approach that allows us to study the above processes at the single molecule level in egg extracts. Read on to learn more about the projects and people in the Walter lab.