A loxP site was inserted upstream of the first exon and a floxed neo cassette was inserted downstream of exon 1. Germ line, cre-mediated recombination was used to remove the neo cassette leaving exon 1 floxed. In a subsequent publication (J:150427), the authors demonstrate that that the upstream loxP site was inserted in a separate gene, Pcid2, that lies in a head-to-head configuration with Cul4a. This loxP site, inserted in a NotI restriction site, lies downstream of the putative translational start site of Pcid2. Cre-mediated excision can thus potentially disrupt both genes. (J:78032, J:138468, J:150427)