Speaker: Peter Swain Quantifying Fluorescence Measurements in Single Cells Single cell measurements can be essential to determine network responses. For example, an all-or-nothing response may appear as graded at a population level. I will present in vivo measurements of the gene regulation function: the quantitative relation between transcription factor concentrations and the rate of protein production from downstream genes. By studying individual cells, we show that the gene regulation function fluctuates dynamically, thereby limiting the accuracy with which transcriptional genetic circuits can transfer signals. We found that protein production rates fluctuate over a time scale of about one cell cycle, while intrinsic noise decays rapidly. Further, by following the asymmetric partitioning of fluorescence between daughter cells at each division, we are able to estimate molecule numbers, and thus calibrate in vivo biochemical parameters in molecular units. Biochemical parameters and fluctuations, both their size and their lifetime, determine the effective single cell gene regulation function.