FACS analysis of expression of RFP in a cloned cell line. Tetracycline (0.5 μg ml-1) was added and samples were removed at specific timepoints. The level of fluorescence in the population was measured by counting 5 × 103 cells per timepoint on a FACScan. The filled curve is the uninduced population. The traces for induced cells are shown as overlaid lines: blue, 24 hours: red, 48 hours and green, 120 hours post-induction. The Y axis indicates the number of cells counted, whilst the X axis shows the level of RFP expression in AFU.