RNA regulation at the single molecule level: from nuclear organization to molecular activity: Cellular behavior depends on the activities of countless individual biomolecules. Compared to population-based methods, the ability to simultaneously perturb and analyze individual molecules provides a more direct and quantitative understanding, and unprecedented insights, of how these molecules function. I am passionate about performing these single molecule studies in individual mammalian cells, both to quantitatively investigate important but currently inaccessible molecular and cellular biology questions and to achieve more predictive cellular manipulation. In this seminar, I will show two examples: in the first part, I will introduce a FISH-based method to quantitatively measure co-transcriptional splicing efficiency at the single-cell level and revealed a new functional role of ubiquitous RNA splicing as a gene expression filter acting in an ‘economy of scale’ manner; in the second part, I will elaborate on a magnetic-tweezer-based method for quantifying the kinetics of competitive nucleic acid hybridization systematically and revealing its mechanistic differences from conventional hybridization.