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What is driving the remarkable increase over the last decade in the propensity of patents to cite academic science? Does this trend indicate that stronger knowledge spillovers from academia have helped power the surge in innovative activity in the U.S. in the 1990s? This paper seeks to shed light on these questions by using a common empirical framework to assess the relative importance of various alternative hypotheses in explaining the growth in patent citations to science. Our analysis supports the notion that the nature of U.S. inventive activity has changed over the sample period, with an increased emphasis on the use of the knowledge generated by university-based scientists in later years. However, the concentration of patent-to-paper citation activity within what we call the "bio nexus" suggests that much of the contribution of knowledge spillovers from academia may be largely confined to bioscience-related inventions.