The Cre-loxP system has become an important strategy for conditional gene deletion and conditional gene expression in genetically engineered mice. To evaluate Cre recombinase expression, we generated reporter mice that permit both noninvasive imaging in living animals and either ex vivo histochemical/immunohistochemical tissue transgene expression analysis or quantitative enzyme analysis in the same animal.Transgenic reporter mice were generated in which a loxP-flanked enhanced green fluorescent protein (EGFP) reporter gene and STOP sequence are placed after the nearly ubiquitously expressed CAG promoter, but before a bicistronic transcriptional unit containing luciferase and β-galactosidase reporter gene coding sequences.After global deletion of the floxed STOP sequence by germ line Cre deletion, the reporter mouse expresses luciferase and β-galactosidase in all tissues examined. Tissue-specific expression of both reporter genes occurs in reporter mouse strains expressing Cre in skin (K14 keratin Cre), heart (myosin light chair Cre), or colon (Villin Cre).The luc-gal(Tg) reporter mouse allows noninvasive imaging of target Cre activation both in living animals and in tissues and cells following necropsy, using loss of EGFP expression, gain of luciferase expression, and gain of β-galactosidase expression as alternatives within the same animal for qualitative analysis of Cre expression.