Fluorescence imaging of gene expression. (A) The structure of GFP. (B) A transgenic GFP mouse. (C) Real-time whole-body imaging of a GFP-expressing human glioma growing in the brain of a nude mouse at 1 wk (left), 3 wk (center), and 5 wk (right) after surgical orthotopic implantation. (D) Monomeric and tandem dimeric fluorescent proteins derived from Aequorea GFP or Discosoma RFP, expressed in bacteria, and purified. (Adapted, from Shaner et al. 2004; Hoffman and Yang 2006 and reprinted with permission from Macmillan © 2004, 2006.)