Signal intensity measurements of controls. (a) Microscopy images for negative (scrambled) and positive controls (CD81) in the hepatitis C virus infection screen. The left panel shows the nuclei (DAPI) channel, the right panel the virus signal (GFP). A clear effect of the CD81 knockdown showing significant reduction of the GFP signal is visible, while cell counts are not affected. (b) Histogram of single cell log-transformed GFP intensity values of the positive and negative controls in the HCV screen, measuring hepatitis C virus infection efficiency. The brown curve indicates the signal distribution of all cells in the entire screen, perfectly matching the histogram of the negative, but not of the positive controls. Notably, two populations of cells are discernible both in the positive and negative controls, corresponding to infected and non-infected cells. Positive and negative controls differ in the sizes of these subpopulations, and thus the shape of the overall distribution of GFP intensity values.