Genome stability relies on a network of interacting systems that detect and repair disturbances in the integrity of DNA. A central pathway that processes single-stranded DNA gaps and potentially lethal double-strand breaks in DNA that can occur during DNA replication employs a mechanism that repairs these lesions by homologous recombination. Primary components of this pathway in eukaryotes include Rad51 whose function is to search for DNA sequence homology and promote strand exchange, plus factors that enable, enhance, and control Rad51's activity. Among this constellation of Rad51-interacting proteins the master regulator is BRCA2, the product of a breast cancer susceptibility gene. BRCA2 in turn, is completely dependent upon its key interacting partner, Dss1, for activity. Through the concerted workings of this hierarchical complex comprised of Dss1, BRCA2, and Rad51, the homologous recombination system is harnessed to maintain stability and preserve the integrity of the genome. We seek to understand how BRCA2 function is regulated by Dss1 and to understand the role of BRCA2 in governing the activity of Rad51. We use the microbe Ustilago maydis as a model system for experimentation because it has a well-conserved BRCA2-homolog, Brh2, and is amenable to molecular genetic manipulations and analysis. The powerful attributes of this system open the way for gaining insight into BRCA2's molecular mechanism through avenues not immediately approachable in the vertebrate systems. Our primary objectives are to investigate the role of Dss1 as a regulator of Brh2, to examine how Brh2 controls the activity of Rad51, and to investigate how select proteins interact genetically with Brh2 and Rad51 to regulate homologous recombination activity. Our long-term goal is to provide knowledge of the function of BRCA2 and gain insight into how its dysfunction can lead to initiation of tumorigenic transformation.