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<jats:title>Toward Drug Development</jats:title> <jats:p> The vast majority of pharmaceuticals on the market are based on natural product molecules originally derived from living organisms. When it comes to newly approved drugs, however, the situation looks rather different. Difficulties with high-throughput screening and laboratory synthesis of natural products have led drug companies to focus on libraries of synthetic compounds, despite their providing a much lower “hit rate.” <jats:italic>Li and Vederas</jats:italic> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="161" related-article-type="in-this-issue" vol="325" xlink:href="10.1126/science.1168243">161</jats:related-article> ) review methodologies that facilitate the screening, analysis, and synthesis of natural product molecules and their derivatives. Given these advances and the vast numbers of organisms and environments that remain to be explored for potential drug candidates, the current lull in newly approved drugs based on natural products will likely be temporary. </jats:p>

<jats:p>Advancing spinal neuron growth cones generate traction forces that can direct the trajectory of the axon.</jats:p>

<jats:title>Size Matters</jats:title> <jats:p> Cells of different types come in different sizes. Size is established by a trade-off of growth and division; as a result, the distribution of cell size in a population is held stable over time. A classic question in cell physiology is whether the growth rate of cells is constant over time or whether, as cells grow bigger during the cell cycle, they grow faster and faster. Using experimental and mathematical approaches, <jats:bold> Tzur <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="167" related-article-type="in-this-issue" vol="325" xlink:href="10.1126/science.1174294">167</jats:related-article> ; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="158" related-article-type="in-this-issue" vol="325" xlink:href="10.1126/science.1177203">Edgar and Kim</jats:related-article> </jats:bold> ) show that the growth rate of mouse lymphoblastoid cells in culture is slow in the G <jats:sub>1</jats:sub> phase and then increases to reach a constant exponential rate. Thus, there is an active size-control mechanism that limits size variation in animal cells. </jats:p>

<jats:title>Sunspots Before Your Eyes</jats:title> <jats:p> Sunspots have a dark central region, the umbra, surrounded by a region of lighter radial filaments, the penumbra, along which there are outward horizontal mass flows that are still not fully understood despite their discovery 100 years ago. Now <jats:bold> Rempel <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="171" related-article-type="in-this-issue" vol="325" xlink:href="10.1126/science.1173798">171</jats:related-article> , published online 18 June; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="155" related-article-type="in-this-issue" vol="325" xlink:href="10.1126/science.1175752">Scharmer</jats:related-article> </jats:bold> ) present comprehensive numerical simulations of a pair of sunspots that show the development of an outer penumbra with systematic radial outflows along channels of nearly horizontal magnetic field in regions where the average field inclination is greater than 45 degrees. The outflows result from rising hot plasma that turns over and is guided outward by the strong and inclined magnetic field. The simulations reproduce observed properties throughout the entire penumbra and show that the penumbral structure and outflows in sunspots can be understood in terms of convective flow in a magnetic field with varying inclination. </jats:p>

<jats:title>Strolling Out on a Quantum Walk</jats:title> <jats:p> In a random walk, a walker moves one step to the left or one step to the right depending on the outcome of a coin toss. The distribution between possible locations is well known and forms the basis for algorithms in information processing, describing diffusion processes in physics or biology, and has even been used as a model for stock market prices. <jats:bold> Karski <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="174" related-article-type="in-this-issue" vol="325" xlink:href="10.1126/science.1174436">174</jats:related-article> ) use a single caesium atom trapped in a one-dimensional optical lattice to implement the quantum counterpart—a quantum walk. The coherence of a quantum system results in a departure from the classical picture, producing a distribution that is quite different that depends on the internal state of the atom. The results may have implications for search algorithms and quantum information processing protocols. </jats:p>

<jats:title>Topological Insulators</jats:title> <jats:p> Topological insulators are a recently discovered state of matter, in which the bulk is an insulator while the surface is metallic with counterpropagating spin states. The surface states are protected by the topology, or structure, of the Fermi surface in the bulk gap and are described by a Dirac cone showing linear dispersion behavior meeting at the Dirac point. <jats:bold> Chen <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="178" related-article-type="in-this-issue" vol="325" xlink:href="10.1126/science.1173034">178</jats:related-article> , published online 11 June) provide a comprehensive photoemission study on Bi <jats:sub>2</jats:sub> Te <jats:sub>3</jats:sub> showing that it too falls into the category of topological band insulators. Moreover, there is just a single surface state with a single Dirac point in the photoemission spectrum. The identification of a material with a single Dirac point removes the ambiguity arising from multiple surface states and provides an ideal test-bed to probe the physics of these exotic new materials. </jats:p>

<jats:title>Watching Graphite Breathe</jats:title> <jats:p> Electron microscopy is best known for its capacity to resolve spatial features. In a variant of the technique termed electron energy loss spectroscopy (EELS), the kinetic energy dissipated as the electron probe beam penetrates a sample can be quantified to characterize the material's underlying electronic structure. <jats:bold> Carbone <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="181" related-article-type="in-this-issue" vol="325" xlink:href="10.1126/science.1175005">181</jats:related-article> ) have achieved femtosecond time resolution in an EELS study of graphite, and thereby traced the correlated motion of electrons and nuclei when the sample is heated by laser irradiation. The laser pulse induces rapid compression and then expansion of the layers in graphite's sheet structure. Concurrently, EELS uncovers shifts in delocalized electronic excitations, or plasmons. Layer compression coincides with a population shift from surface to bulk (that is, interlayer) plasmons, with the reverse shift accompanying expansion. </jats:p>

<jats:title>Electrons Accepted Here</jats:title> <jats:p> Methane is produced in large quantities in marine sediments during the breakdown of organic matter. Methane is a powerful greenhouse gas that plays a large role in the regulation of climate. Methane is also an energy source for the abundant anaerobic methanotrophs that consume most of it before it ever reaches the atmosphere. The anaerobic oxidation of methane in marine systems depends on the presence of sulfate, which acts as an electron acceptor and is often considered essential for the reaction to proceed. <jats:bold> Beal <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="184" related-article-type="in-this-issue" vol="325" xlink:href="10.1126/science.1169984">184</jats:related-article> ) report that anaerobic methane oxidation in marine sediments can be facilitated by iron and manganese, as well as by sulfate. Thus, anaerobic methane oxidation using iron and manganese could have been an important methane sink, and energy source, for the early biosphere. </jats:p>

<jats:title>Cooling Down</jats:title> <jats:p> Some aerosols, such as sulfates, reflect solar radiation and have a cooling effect on climate, while others, such as black carbon, have a warming effect because they absorb solar radiation. The preponderance of reflective aerosols causes the net effect to be one of cooling, but the amount of cooling is uncertain, owing to a large difference in estimates of the effect of aerosols in global aerosol models compared with estimates based on observations. <jats:bold>Myhre</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="187" related-article-type="in-this-issue" vol="325" xlink:href="10.1126/science.1174461">187</jats:related-article> , published online 18 June; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="153" related-article-type="in-this-issue" vol="325" xlink:href="10.1126/science.1176991">Quaas</jats:related-article> </jats:bold> ) uses a combination of observational data and modeling to reconcile the two approaches, drawing particular attention to relative increases in the aerosol fraction of black carbon. Taken together, the results suggest a best estimate of the cooling effect of aerosols that is 60% of previous reports. </jats:p>

<jats:title>Parkfield Tremors</jats:title> <jats:p> Parkfield, California, sits on the San Andreas Fault near the end of a major historic rupture in 1857. Recent monitoring has resolved a change in the seismic tremor—small repeating earthquakes that have been occurring in certain parts of the fault. <jats:bold>Nadeau and Guilhem</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="191" related-article-type="in-this-issue" vol="325" xlink:href="10.1126/science.1174155">191</jats:related-article> ) now show that the tremors increased and periodic episodes began around the time of two moderate nearby earthquakes 4 years ago. Surprisingly, the tremor episodes have persisted, rather than decaying after the quakes, which may imply that there has been a step change in the state of stress on this part of the San Andreas Fault. </jats:p>