This allele is used to detect transient expression of Cre recombinase. The transgene carries a lacZ gene downstream of a CMV enhancer and chicken beta-actin promoter and a floxed chloramphenicol acetyltransferase gene (a stuffer sequence). Upon Cre-mediated excision of the stuffer sequence, the modified allele expresses lacZ in tissues where Cre is expressed. The authors state that a single copy of the transgene was inserted and expression was observed throughout all embryonic and adult stages.