mus312 mutants have greatly decreased rates of crossing over, about 5% of the wild-type level. The decrease is the same in every genetic interval tested, like mei-9. This led us to beleive that MUS312 functions at the step in meiotic recombination as MEI-9. We mapped mus312 to a small region of the genome containing about 50 predicted genes. At that point, a yeast two-hybrid screen for proteins that interact with MEI-9 fortuitously yielded a clone corresponding to one of these 50 genes! We sequenced the open reading frame from each of three mutant alleles of mus312 and discovered nonsense mutations in each (Yildiz et al., 2003). For more, see

Ö. Yildiz, S. Majumder, B.C. Kramer, and J. Sekelsky (2002) Drosophila MUS312 protein interacts physically with the nucleotide excision repair endonuclease MEI-9 to generate meiotic crossovers. Molecular Cell 10: 1503-1509.
 

The predicted MUS312 protein is completely novel, but we do know that it physically interacts with MEI-9 to generate meiotic crossovers. We have since identified putative orthologs in other insects, vertebrates, and even a few fungi.

mus312 mutants are like mei-9 mutants in another aspect: acute hypersensitivity to the DNA crosslinking agent nitrogen mustard. This suggests that the MUS312 protein plays a role in the poorly understood process of interstrand crosslink repair. Several anti-cancer drugs (e.g. cisplatin, mitomycin C, nitrogen mustard) generate interstrand crosslinks, so it is important that the pathway for repair of such lesions be elucidated.