it has been suggested that GFP is dimeric in vivo (M.W.Cutler and W.Ward,
Photochem. Photobiol. 57, 63S (1993).
However, dimerization is not necessary for GFP to fluoresce. Time
resolved fluorescence is not influenced by cryoprotectors which supress
dimerization (H. Lossau et al., Chemical Physics 2653 (Dec. 1996)).