The transgene comprises the CMV enhancer and chicken beta-actin promoter and the beta-galactosidase coding sequence separated by the chloramphenicol acetyltransferase (cat) gene flanked by loxP sites. Beta-galactosidase expression is blocked by presence of the cat gene; when the CAT sequence is excised by Cre recombinase, beta-galactosidase is expressed. (J:66884, J:80158)