An IRES-EGFP-cre cassette and FRT-flanked neo marker were inserted into a BAC containing the Spo11 gene. A fragment containing the Spo11 sequence with 5 kb of upstream sequence, the IRES-EGFP-cre cassette and neo marker was isolated from the modified BAC and transfected into ES cells. Targeted ES cells were injected into blastocysts to generate transgenic animals. Founder information and line numbers are not available.