A 3.4 kb fragment including all coding exons (exons 2- 5) was flanked with loxP sites and an adenoviral splice acceptor site fused to a cDNA encoding eGFP was inserted downstream of the second loxP site. In vivo cre-mediated recombination resulted in deletion of exons 2-5 and expression of eGFP under control of the endogenous promoter.