Donor construct for gene targeting at the y locus. A 6.7kb segment of the y gene, in which the zinc finger nuclease (ZFN) recognition site which is recognised by Zzzz\ZFNhs.yA and Zzzz\ZFNhs.yB in combination has been replaced by two in-frame stop codons and a XhoI site. The y sequence is flanked by FRT and RS(I-SceI) sites.