In eukaryotes, homologous recombination (HR) is a major pathway for the repair of DNA double-stranded breaks (DSBs). Failures in DSB processing and repair can lead to genomic instability and a predisposition to various types of cancers. DNA helicases play important roles in this process, and mutations in several FIR-specific helicases e.g. BLM, WRN, and RTS are associated with cancer-prone human syndromes. Therefore, understanding the role of DNA helicases in FIR at the mechanistic level has direct relevance to delineating cancer etiology. Genetic, cytological, biochemical and other analyses of a number of DNA helicases have revealed their roles in HR regulation. Some of these DNA helicases can act at an early stage in IIR to prevent inappropriate recombination events. For example, the S. cerevisiae Srs2 and human RecQ5 helicases serve this function. DNA helicases also act at later stages of HR to prevent crossover product and chromosomal translocations and loss of heterozygosity that could result from crossover types of HR. Moreover, human BTM together with Topo III α, BLAP75 and BLAP18 can dissolve the double Holliday junction, a DNA intermediate generated during HR, to produce non-crossover products. During mitosis, crossover products are disfavored by the usage of the Synthesis Dependent Strand Annealing (SDSA) pathway. However, the DNA helicase that performs this reaction has remained unknown. The main goal of my thesis project has been to provide insight into the role of the Mph1 helicase in the SDSA pathway of HR. Procedures for the expression and purification of Mph1 and various mutant forms have been devised. Purified Mph1 possesses ssDNA-dependent ATPase and 3'-5' DNA helicase activities, and chromatin immunoprecipitation has provided evidence for the DSB recruitment of this helicase. A variety of mechanistic studies have shown that Mph1 binds and unwinds the D-loop structure, the first DNA joint formed between recombining DNA molecules in cells. Overall, my research results furnish evidence to implicate Mph1 in the promotion of the SDSA type of HR via D-hop dissociation. These results have important implications as to the biological role of the Mph1 orthologue FANCM, which is mutated in the cancer-prone disease Fanconi anemia, complementation M.