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<jats:title>Exploring photonic topology</jats:title><jats:p>Scattering topological effects are being explored in a variety of electronic and optical materials systems owing to their robustness against defects (see the Perspective by Özdemir). Yang<jats:italic>et al.</jats:italic>designed and fabricated an ideal optical analog of a three-dimensional Weyl system. Angular transmission measurements revealed four Weyl points at the same energy, as well as the signature helicoidal arcs associated with such an exotic topological system. Zhou<jats:italic>et al.</jats:italic>theoretically proposed and experimentally demonstrated the formation of a topologically protected bulk Fermi arc. They attributed the formation of the arc to the topological nature of paired exceptional points (points at which gain and loss in the system are matched). Photonic crystals may provide a powerful platform for studying exotic properties of topological electronic systems and may also be used to develop optical devices that exploit topological properties of light-matter interactions.</jats:p><jats:p><jats:italic>Science</jats:italic>, this issue p.<jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6379" page="1013" related-article-type="in-this-issue" vol="359" xlink:href="10.1126/science.aaq1221">1013</jats:related-article>, p.<jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6379" page="1009" related-article-type="in-this-issue" vol="359" xlink:href="10.1126/science.aap9859">1009</jats:related-article>; see also p.<jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6379" page="995" related-article-type="in-this-issue" vol="359" xlink:href="10.1126/science.aar8210">995</jats:related-article></jats:p>

<jats:title>Exploring photonic topology</jats:title> <jats:p> Scattering topological effects are being explored in a variety of electronic and optical materials systems owing to their robustness against defects (see the Perspective by Özdemir). Yang <jats:italic>et al.</jats:italic> designed and fabricated an ideal optical analog of a three-dimensional Weyl system. Angular transmission measurements revealed four Weyl points at the same energy, as well as the signature helicoidal arcs associated with such an exotic topological system. Zhou <jats:italic>et al.</jats:italic> theoretically proposed and experimentally demonstrated the formation of a topologically protected bulk Fermi arc. They attributed the formation of the arc to the topological nature of paired exceptional points (points at which gain and loss in the system are matched). Photonic crystals may provide a powerful platform for studying exotic properties of topological electronic systems and may also be used to develop optical devices that exploit topological properties of light-matter interactions. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6379" page="1013" related-article-type="in-this-issue" vol="359" xlink:href="10.1126/science.aaq1221">1013</jats:related-article> , p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6379" page="1009" related-article-type="in-this-issue" vol="359" xlink:href="10.1126/science.aap9859">1009</jats:related-article> ; see also p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6379" page="995" related-article-type="in-this-issue" vol="359" xlink:href="10.1126/science.aar8210">995</jats:related-article> </jats:p>

<jats:title>Guiding nitrenes away from a migration</jats:title> <jats:p> Nitrogen conventionally shares its electrons in three bonds with one or more partners. A singly bonded nitrogen, or nitrene, is exceptionally reactive and can insert itself into normally inert C–H bonds. If the nitrene forms next to a carbonyl center, though, it tends to react with the C–C bond on the other side instead. Hong <jats:italic>et al.</jats:italic> used theory to guide the design of an iridium catalyst that inhibits this rearrangement, steering the nitrene toward C–H insertion to form a variety of useful lactam rings. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6379" page="1016" related-article-type="in-this-issue" vol="359" xlink:href="10.1126/science.aap7503">1016</jats:related-article> </jats:p>

<jats:title>Mudrocks get a vegetative assist</jats:title> <jats:p>Mudrocks such as slate and shale are rarely found in stratigraphy older than about 500 million years. McMahon and Davies compiled a large database of mudrock occurrence over the past 3.5 billion years to help assess the origin of this ubiquitous rock type (see the Perspective by Fischer). Mudrocks appeared at the same time as did deep-rooted land plants. The interplay between plants and sedimentary rocks suggests that a change in erosion rate and the chemistry of sediments delivered to the oceans occurred around 500 million years ago.</jats:p> <jats:p> <jats:italic>Science, this issue</jats:italic> p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6379" page="1022" related-article-type="in-this-issue" vol="359" xlink:href="10.1126/science.aan4660">1022</jats:related-article> ; see also p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6379" page="994" related-article-type="in-this-issue" vol="359" xlink:href="10.1126/science.aas9886">994</jats:related-article> </jats:p>

<jats:title>How to select and shape neural activity</jats:title> <jats:p> When we learn a new skill or task, our movements are reinforced and shaped. Learning occurs because the neural activity patterns in the movement control–related brain regions that are rewarded are repeated. But how does this reinforcement work? Athalye <jats:italic>et al.</jats:italic> developed a closed-loop self-stimulation paradigm in which a target motor cortical activity pattern resulted in the optogenetic stimulation of dopaminergic neurons. With training, mice learned to reenter specific neuronal activity patterns, which triggered self-stimulation and shaped their neural activity to be closer to the target pattern. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6379" page="1024" related-article-type="in-this-issue" vol="359" xlink:href="10.1126/science.aao6058">1024</jats:related-article> </jats:p>

<jats:title>Ratcheting up wild virulence</jats:title> <jats:p> Partially protective vaccination can sometimes select for increasingly virulent pathogens. Fleming-Davies <jats:italic>et al.</jats:italic> asked what happens in a natural system. In the United States, the house finch population is suffering an increasingly virulent epidemic caused by <jats:italic>Mycoplasma gallisepticum</jats:italic> . The pathogen induces incomplete immunity that clears less virulent pathogens and offers partial protection against strains of greater virulence. In the birds, the partial immune response does away with competition from the less virulent pathogens. The partial immunity of the host also hinders replication of the more virulent pathogens enough to allow some birds to survive. This allows increasingly virulent forms of the pathogen to be transmitted. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6379" page="1030" related-article-type="in-this-issue" vol="359" xlink:href="10.1126/science.aao2140">1030</jats:related-article> </jats:p>

<jats:title>Changing coats with the season</jats:title> <jats:p> Many species of mammals and birds molt from summer brown to winter white coats to facilitate camouflage. Mills <jats:italic>et al.</jats:italic> mapped global patterns of seasonal coat color change across eight species including hares, weasels, and foxes. They found regions where individuals molt to white, brown, and both white and brown winter coats. Greater proportions of the populations molted to white in higher latitudes. Regions where seasonal coat changes are the most variable (molting to both brown and white) may provide resilience against the warming climate. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6379" page="1033" related-article-type="in-this-issue" vol="359" xlink:href="10.1126/science.aan8097">1033</jats:related-article> </jats:p>

<jats:title>Engineering cytokine-receptor pairs</jats:title> <jats:p> Interleukin-2 (IL-2) is an important cytokine that helps T cells destroy tumors and virus-infected cells. IL-2 has great therapeutic promise but is limited by toxic side effects and its capacity to both activate and repress immune responses. Sockolosky <jats:italic>et al.</jats:italic> set out to improve IL-2–based immunotherapy by engineering synthetic IL-2–receptor pairs (i.e., IL-2 and its receptor, IL-2R) (see the Perspective by Mackall). Engineered complexes transmitted IL-2 signals but only interacted with each other and not with endogenous IL-2/IL-2R. Treatment of mice with IL-2 improved the ability of engineered T cells to reject tumors with no obvious side effects. This type of approach may provide a way to mitigate toxicities associated with some cytokine-based immunotherapies. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6379" page="1037" related-article-type="in-this-issue" vol="359" xlink:href="10.1126/science.aar3246">1037</jats:related-article> ; see also p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6379" page="990" related-article-type="in-this-issue" vol="359" xlink:href="10.1126/science.aas9434">990</jats:related-article> </jats:p>