Donor construct for targeting of the Gr32a locus by homologous recombination. The construct is designed so that Gr32a sequences should be replaced with a mini-w marker upon homologous recombination. The construct contains a region from upstream of Gr32a (coordinates 11110716-11114171) and a region from downstream of Gr32a (coordinates 11115121-11118790) which flank a mini-w marker with an RS(I-SceI) site. The entire construct is contained within an FRT cassette.