The chemiluminescence assay is linear. Extracts from MELc populations in which all the cells expressed the LacZ gene (cells with cassette 234 at RL1) were sequentially diluted with extracts from nonexpressing cells and the levels of β-gal expression were measured using the chemiluminescence assay. Extracts from uninduced, hemin-, or DMSO-induced cells were used in three independent experiments. The means of the levels of β-gal expression (normalized to the 100% positive cells extract) obtained for all three types of extract is plotted against the percentage of positive cells per extracts. This shows that the chemiluminescence assay is almost perfectly linear over the range of interest. As shown by the small error bars, results for each type of extract plotted individually were similar.