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<jats:p> Comet Hale-Bopp is an unusually large and bright comet that will reach perihelion (closest approach to the Sun) in April 1997. In his Perspective, Cruikshank discusses observational results described in seven research reports in this issue of <jats:italic>Science</jats:italic> (see pages <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1900" related-article-type="in-this-issue" vol="275" xlink:href="10.1126/science.275.5308.1900" xlink:type="simple">1900</jats:related-article> , <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1904" related-article-type="in-this-issue" vol="275" xlink:href="10.1126/science.275.5308.1904" xlink:type="simple">1904</jats:related-article> , <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1907" related-article-type="in-this-issue" vol="275" xlink:href="10.1126/science.275.5308.1907" xlink:type="simple">1907</jats:related-article> , 1909, <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1913" related-article-type="in-this-issue" vol="275" xlink:href="10.1126/science.275.5308.1913" xlink:type="simple">1913</jats:related-article> , <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1915" related-article-type="in-this-issue" vol="275" xlink:href="10.1126/science.275.5308.1915" xlink:type="simple">1915</jats:related-article> , and 1918). Images from the Hubble Space Telescope indicate a large cometary nucleus, and measurements from the Infrared Satellite Observatory craft reveal the composition of the dust grains being discharged by Hale-Bopp. Radio telescope measurements have tracked the release of carbon, oxygen, hydrogen, nitrogen, and sulfur in various molecular species. Photometry shows that gas production is 20 to 100 times greater than any previously observed comet. Such observations may help unravel the puzzle of comet formation and the origins of organic matter in the universe. </jats:p>

<jats:p> Chemists and physicists have recently joined forces to create transistor devices on a nanometer scale. In his Perspective, Kouwenhoven discusses results reported by Bockrath <jats:italic>et al</jats:italic> . in the same issue (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1922" related-article-type="in-this-issue" vol="275" xlink:href="10.1126/science.275.5308.1922" xlink:type="simple">1922</jats:related-article> ) in which carbon nanotubes were used to create switching elements that behave like transistors. Such devices are completely governed by the laws of quantum mechanics and open the possibilities for fundamentally different kinds of circuits at enormously higher density. </jats:p>

<jats:p> The vast epithelial lining of the gut has its own branch of the immune system to protect the body from invasion. In his Perspective, Shanahan describes the components of this system—the intestinal epithelial cells and intraepithelial lymphocytes—and a new local paracrine network among them, mediated by thyrotropin-releasing hormone and thyroid-stimulating hormone and reported in this issue on p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1937" related-article-type="in-this-issue" vol="275" xlink:href="10.1126/science.275.5308.1937" xlink:type="simple">1937</jats:related-article> (Wang <jats:italic>et al</jats:italic> .) </jats:p>

<jats:p> The enzyme PI 3-kinase seems to mediate extraordinarily diverse cellular responses to growth factors. In his Update Perspective, Hemmings offers an explanation by outlining the evidence for a common target of the phospholipid signaling molecules generated by PI 3-kinase—the pleckstrin homology (PH) domains located on at least 100 cellular proteins, which could mediate the diverse responses of cells. On p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1927" related-article-type="in-this-issue" vol="275" xlink:href="10.1126/science.275.5308.1927" xlink:type="simple">1927</jats:related-article> of this issue, Klarlund <jats:italic>et al</jats:italic> . report the isolation of one of these PH domain-containing proteins, GRP1. </jats:p>