for single-cell mass-spectrometry analysis promise to enable studying signal transduction with single-cell resolution. in notch signaling, direct contact between cells allows for precise control of cell differentiation during embryonic development. in the worm caenorhabditis elegans, two cells of the developing gonad each have an equal chance of terminally differentiating or becoming a uterine precursor cell that continues to divide. the choice of which cell continues to divide is controlled by competition of cell surface signals. one cell will happen to produce more of a cell surface protein that activates the notch receptor on the adjacent cell. this activates a feedback loop or system that reduces notch expression in the cell that will differentiate and that increases notch on the surface of the cell that continues as a stem cell.