The transgene comprises the following elements: the CAG promoter, a loxP site, a bacterial chloramphenicol acetyltransferase (cat) reporter gene, poly(A) features, a loxP site and the mouse serum response factor (Srf) cDNA. The inserted mouse gene will only be expressed after removal of the cat gene through cre-mediated recombination. This line was selected for its low level expression. (J:243527)