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Biochemical form is shaped by biological function. Biochem- ical investigation of enzymes involved in eukaryotic homolo- gous genetic recombination now is progressing rapidly, having only recently been inaugurated with the study of Rad51 protein, a homolog of the bacterial RecA protein. A significant new front is opened with the first report on the in vitro activities of a second and meiosis-specific RecA homolog, the human Dmc1 protein (1). Results on Rad51 and Dmc1 to date have yielded a number of surprises in the form of some intriguing and perhaps unanticipated differences with respect to the RecA protein. Pondering biochemical differences must inev- itably bring one back to a fundamental question: why do cells recombine DNA? The history of in vitro research into mechanisms of genetic recombination now spans two decades. Until quite recently, bacteria have provided most of the recombination enzymes and most of the biochemical insight. The bacterial RecA protein has been at the center of this activity. Little genetic information is exchanged between bacterial chromosomes without RecA. The RecA protein aligns two DNA molecules, facilitates a strand switch that generates a crossover between them, and promotes migration of the resulting DNA branch. An ever-growing host of additional proteins prepare DNA substrates for RecA action, modulate RecA binding to DNA, or process the branched DNA recombination intermediates created by RecA (2-4). A parallel universe of recombination enzymes appears to exist in eukaryotes. The curtain opened in 1992 with the demonstration that in Rad51 (5) and Dmc1 (6) yeast possessed at least two RecA homologs. Mammals also possess homologs of both proteins, and their investigation has taken some interesting turns. Targeted disruption of mouse rad51 confers an embryonic lethal phenotype and sensitivity to ionizing radiation (7, 8). Several proteins involved in carcinogenesis interact with Rad51, including p53 (9), BRCA1 (10), and BRCA2 (11). These results have lent some urgency to the in vitro investigation of Rad51. Following the pioneering work of Sung (12), this now is being pursued in more than a dozen laboratories worldwide. The Dmc1 protein of yeast colocalizes with Rad51 on the zygotene chromosomes (13) and also must play an important role in recombination.

Recombinational crossroads: Eukaryotic enzymes and the limits of bacterial precedents