A hypomorphic allele was generated by the replacement of exon 27 (coding nt 9418 through 9984) with an HPRT minigene. RT-PCR analysis identified a transcript lacking the targeted sequence encoding the RAD51-interacting domain. Truncated protein is translated and the ability of the protein to bind RAD51 is retained, putatively via the BRC motifs encoded by exon 11. The mutant protein contains more than 94% of the wild-type amino acid sequence.