They suggested that this was the first study in which the effect of cell distribution on gene transfer was directly assessed. The GFP allowed them to visualize transconjugant cell distributions in situ, which had not previously been possible. Christensen et al. (1996) investigated details on the distribution of cells active in conjugation. Their approach was to monitor the flux of the gfp-tagged TOL plasmid transfer between two Pseudomonas strains established in a biofilm community. Although they observed plasmid transfer, the fraction of GFP1 cells was small, and the signal was faint. However, they suggested that the GFP system offers great potential for gene transfer studies due to the simple detection and high sensitivity of the GFP signal.