The DNA damage response in eukaryotes involves multiple, complex, and often redundant pathways that respond to various types of DNA damage that affect one or both strands of DNA. One type of toxic DNA damage that can occur is a double-strand break (DSB). Repair of a DSB can lead to the formation of a recombination product known as a crossover (CO). Crossovers in mitotic cells can be deleterious and lead to chromosomal rearrangements or cell death. In order to limit crossing over during DSB repair, eukaryotes possess mechanisms to ensure crossovers do not occur. In this manner, several helicases function during repair of DSBs to promote accurate repair and prevent the formation of crossovers through homologous recombination. Among these helicases is the Fanconi anemia group M (FANCM) protein. FANCM is one of 17 Fanconi anemia (FA) proteins and is one of the most broadly? conserved FA proteins. FANCM and its orthologs, Mph1 and Fml1, are DNA junction-specific helicases/translocases that process homologous recombination (HR) intermediates. Additionally, FANCM has been implicated in a number of DNA metabolic processes including activation of the S-phase checkpoint, trasversal of interstrand crosslinks, recruitment of the proteins such as the FA core complex and Blm to sites of DNA damage, and prevention of mitotic crossovers during double-strand break repair. The helicase activity of FANCM is believed to be important in crossover prevention, but no helicase activity has been detected in vitro. I report here a genetic and biochemical study of Drosophila melanogaster Fancm. I show that purified Fancm is a 3ʹ to 5ʹ ATP-dependent helicase that can disassemble recombination intermediates, but only through limited lengths of duplex DNA. Using transgenic flies expressing full-length or truncated Fancm, each with either a wild-type or mutated helicase domain, I found that there are helicase-independent and C-terminus independent functions in responding to DNA damage and in preventing mitotic crossovers.