You Have Created Transgenic Mice That Carry the Gene Encoding

You have created transgenic mice that carry the gene encoding green fluorescent protein (GFP) under the control of a ubiquitous promoter. The GFP-encoding gene, however, is inactive unless recombination by Cre removes a blocking sequence between the promoter and the gene. These mice have been mated with those harboring the Cre recombinase under the control of the insulin promoter. The Cre recombinase is fused to the estrogen receptor, which keeps the fusion in the cytosol unless it binds to tamoxifen, in which case it allows nuclear entry. After mating, you select adult mice harboring both of these transgenes and inject them with either tamoxifen or water. Which of the following would you expect to observe later as a result when you examine brain and pancreas tissue samples under a fluorescence microscope?

A) GFP signal is observed in the brain in the presence of tamoxifen only.
B) GFP signal is observed in the brain in the absence of tamoxifen only.
C) GFP signal is observed in the pancreas in the presence of tamoxifen only.
D) GFP signal is observed in the pancreas in the absence of tamoxifen only.
E) GFP signal is observed in the brain in either the presence or the absence of tamoxifen.