Swapping Recombination Proteins Crossing over is a means by which organisms create genetic diversity through the mixing of gene complexes. The primary meiotic crossover pathway in budding yeast, mice, nematodes, and plants requires the Msh4–Msh5 heterodimer, which promotes crossovers by blocking anticrossover activities of the Bloom syndrome helicase. However, some fly species, including members of the genus Drosophila , have lost Msh4–Msh5. Kohl et al. (p. 1363 ) now show that Drosophila have evolved a minichromosome maintenance (MCM)–like protein, dubbed mei-MCM, that performs the same function as Msh4–Msh5. Furthermore, these genes appear to have evolved under positive selection, possibly as a result of their repurposing to this novel function.