The ES cell line used to create Fkbp1atm1Zuk (MGI:2158807)was modified so that a floxed neomycin resistance cassette could be found upstream of exon 3 and an additional loxP site was placed in the intron between exons 3 and 4. This involved the elimination of the hprt minigene in the original mutation and the restoration of exons 3 and 4.