The gene trap cassette inserted into intron 5 of the gene. This is predicted to result in a disruption of the majority of the sequence that encodes the helicase domain, loss of the BRCA1 interaction domain, and partial loss of the nuclear localization sequence. The MLH1 interaction domain, encoded within exon 5, remains intact. Western blot analysis confirmed loss of the encoded protein in MEF and testis lysates from homozygous mutant mice.