Neal, 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)). Harald Lossau