Exon 12, encoding the zinc finger domain, and 55bp of flanking sequence from intron 11 and 106bp of flanking sequence from the 3-prime UTR from the CAST/EiJ sequence of Prdm9 was inserted in place of that of the endogenous sequence in B6(Cg)-Tyr/J derived ES Cells and an FRT flanked neomycin selection cassette was inserted 621bp downstream of the end of exon 12. Breeding to a Cre deleter strain excised the neo cassette.