Gene drive systems promote the spread of genetic elements through populations by assuring they are inherited more often than Mendelian segregation would predict (see the figure). Natural examples of gene drive from Drosophila include sex-ratio meiotic drive, segregation distortion, and replicative transposition. Synthetic drive systems based on selective embryonic lethality or homing endonucleases have been described previously in Drosophila melanogaster (1–3), but they are difficult to build or are limited to transgenic populations. In contrast, RNAguided gene drives based on the CRISPR/Cas9 nuclease can, in principle, be constructed by any laboratory capable of making transgenic organisms (4). They have tremendous potential to address global problems in health, agriculture, and conservation, but their capacity to alter wild populations outside the laboratory demands caution (4–7). Just as researchers working with self-propagating pathogens must ensure that these agents do not escape to the outside world, scientists working in the laboratory with gene drive constructs are responsible for keeping them confined (4, 6, 7). Authors: Omar S. Akbari, Hugo J. Bellen, Ethan Bier, Simon L. Bullock, Austin Burt, George M. Church, Kevin R. Cook, Peter Duchek, Owain R. Edwards, Kevin M. Esvelt, Valentino M. Gantz, Kent G. Golic, Scott J. Gratz, Melissa M. Harrison, Keith R. Hayes, Anthony A. James, Thomas C. Kaufman, Juergen Knoblich, Harmit S. Malik, Kathy A. Matthews, Kate M. O'Connor-Giles, Annette L. Parks, Norbert Perrimon, Fillip Port, Steven Russell, Ryu Ueda, Jill Wildonger

The immune system in the intestine is highly adapted to resist invading pathogens while residing peacefully with the abundant and diverse commensal bacteria that colonize the gastrointestinal tract. In turn, bacterial signals shape immunity in the intestine, promoting intestinal homeostasis in part by inducing and expanding specialized regulatory T (Treg) cells that prevent aberrant inflammatory responses to self and environmental stimuli (1). On pages 989 and 993 of this issue, Ohnmacht et al. (2) and Sefik et al. (3), respectively, report the development and function of a subpopulation of Treg cells found primarily in the large intestine, and characterized by expression of the nuclear hormone receptor retinoic acid receptor-related orphan receptor γt (RORγt). This is surprising because RORγt classically promotes the differentiation of T helper 17 (TH17) cells, a population associated with tissue inflammation in many inflammatory diseases (4). Both studies show that microbiota-derived signals induce the expression of RORγt in Treg cells that control intestinal inflammation (see the figure). These findings highlight the diversity of colonic Treg cells, their complex transcriptional programs, and their important role in the intestine. Authors: Ahmed N. Hegazy, Fiona Powrie

Almost 300 years ago, Linnaeus defined our genus Homo (and its species Homo sapiens) with the noncommittal words nosce te ipsum (know thyself) (1). Since then, fossil and molecular biology studies have provided insights into its evolution, yet the boundaries of both the species and the genus remain as fuzzy as ever, new fossils having been rather haphazardly assigned to species of Homo, with minimal attention to details of morphology. Authors: Jeffrey H. Schwartz, Ian Tattersall

Relying on nearly 70 rare books drawn from 21 library collections, Fantastic Worlds chronicles the era of rapid scientific discovery and innovation that gave birth to the literary genre known as science fiction. Reviewer Rachel Gross revels in the exhibition's juxtaposition of science and fantasy: from Jules Verne's tales of unabashed wonder, to Mary Shelley's haunting story of a scientist who pushes the boundaries of nature and taboo. Author: Rachel Gross

On 10 December 1954, Col. John Stapp shot across the grounds of Holloman Air Force Base on a rocket sled, reaching 639 miles per hour and coming to a complete halt in less than 1.37 seconds. This was not a publicity stunt. Stapp, a biophysicist and medical doctor, was conducting an experiment in the emerging scientific field of biomechanics. In Sonic Wind, author Craig Ryan brings Stapp's death-defying feats to life. Reviewer Lee Vinsel praises the book's smooth prose and Ryan's efforts to humanize Stapp, but questions his portrayal of Stapp as a lone genius. Author: Lee Vinsel

A listing of books received at Science during the week ending 21 August 2015.

Authors: Courtney Addison, Samuel Taylor-Alexander

Author: K. Brad Wray

Authors: Ariana Strandburg-Peshkin, Damien R. Farine, Iain D. Couzin, Margaret C. Crofoot

Fried et al. (Reports, 19 December 2014, p. 1510) demonstrated a strong correlation between reaction rate and the carbonyl stretching frequency of a product analog bound to ketosteroid isomerase oxyanion hole mutants and concluded that the active-site electric field provides 70% of catalysis. Alternative comparisons suggest a smaller contribution, relative to the corresponding solution reaction, and highlight the importance of atomic-level descriptions. Authors: Aditya Natarajan, Filip Yabukarski, Vandana Lamba, Jason P. Schwans, Fanny Sunden, Daniel Herschlag

Fried et al. (Reports, 19 December 2014, p. 1510) demonstrate electric field–dependent acceleration of biological catalysis using ketosteroid isomerase as a prototypic example. These findings were not extended to aqueous solution because water by itself has field fluctuations that are too large and fast to provide a catalytic effect. Given physiological context, when water electrostatic interactions are considered, electric fields play a less important role in the catalysis. Authors: Deliang Chen, Tor Savidge

Natarajan et al. and Chen and Savidge comment that comparing the electric field in ketosteroid isomerase’s (KSI’s) active site to zero overestimates the catalytic effect of KSI’s electric field because the reference reaction occurs in water, which itself exerts a sizable electrostatic field. To compensate, Natarajan et al. argue that additional catalytic weight arises from positioning of the general base, whereas Chen and Savidge propose a separate contribution from desolvation of the general base. We note that the former claim is not well supported by published results, and the latter claim is intriguing but lacks experimental basis. We also take the opportunity to clarify some of the more conceptually subtle aspects of electrostatic catalysis. Authors: Stephen D. Fried, Steven G. Boxer

Schadt and Rosling (Technical Comment, 26 June 2015, p. 1438) argue that primer-template mismatches neglected the fungal class Archaeorhizomycetes in a global soil survey. Amplicon-based metabarcoding of nine barcode-primer pair combinations and polymerase chain reaction (PCR)–free shotgun metagenomics revealed that barcode and primer choice and PCR bias drive the diversity and composition of microorganisms in general, but the Archaeorhizomycetes were little affected in the global study. We urge that careful choice of DNA markers and primers is essential for ecological studies using high-throughput sequencing for identification. Authors: Leho Tedersoo, Mohammad Bahram, Sergei Põlme, Sten Anslan, Taavi Riit, Urmas Kõljalg, R. Henrik Nilsson, Falk Hildebrand, Kessy Abarenkov

In Nairobi, GIST Tech-I finalists transformed their ideas into business plans for great social impact Author: Michaela Jarvis

Author: Becky Ham

Author: Sacha Vignieri

Author: Ian S. Osborne

Author: Jelena Stajic

Author: L. Bryan Ray