Figure 1. The advantages of single cell analysis. (A) In this hypothetical experiment, three samples are obtained that contain a protein of interest at 1, 10, or 50 copies per cell as indicated. The average number of protein molecules per cell is 1 for sample 1, and 5 for both samples 2 and 3. (B) When these cell populations are analyzed by Western blotting, samples 2 and 3 will appear identical. (C) When the samples are analyzed by flow cytometry however, one can clearly see that sample 2 contains cells in two distinct populations that are equally represented, while in sample 3, only about 1 in 10 cells has an elevated level of protein. This kind of heterogeneity in the samples could be due to different cell types (i.e. immune cells), or because of all-or-none type signaling responses.