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<jats:p>An integrated pair of Web sites for students and teachers supports genetics and genomics education worldwide.</jats:p>

<jats:p> A monthly roundup of recent news and projects of <jats:italic>Science</jats:italic> 's publisher, the American Association for the Advancement of Science. </jats:p>

<jats:title>In the (Stem Cell) Zone</jats:title> <jats:p> Efforts to isolate and characterize various types of stem cells have revealed much about the molecular mechanisms of stem cell self-renewal and differentiation, as well as the function of the microenvironment, or stem cell niche. <jats:bold>Li and Clevers</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="542" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1180794">542</jats:related-article> ) review what is known about stem cells in the hair follicle, bone marrow, and intestinal epithelium and suggest that two stem cell populations, one that is quiescent and one that is actively in the cell cycle, may coexist in these tissues in a so-called “zoned” stem cell model. </jats:p>

<jats:p>The hormone pathway that stabilizes seeds may have served more primitive seedless plants in supporting desiccation tolerance.</jats:p>

<jats:title>Promoting Axon Formation</jats:title> <jats:p> How do neurons initiate one axon and lots of dendrites? Using an in vitro assay involving stripes of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), <jats:bold> Shelly <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="547" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1179735">547</jats:related-article> ) show that an increase in cAMP initiates axon formation while an increase in cGMP initiates dendrites. Moreover, cAMP and cGMP reciprocally inhibit each other via the activation of specific phosphodiesterases, as well as protein kinase A and protein kinase G. Finally, long-range self-inhibition of cAMP can explain why only one axon, yet multiple dendrites, is initiated in single hippocampal neurons in culture. Locally increasing cAMP in one neurite causes long-range cAMP reduction in the rest of the neurites, accompanied by corresponding reciprocal changes in cGMP. </jats:p>

<jats:title>Phase Transitions on Carbon Nanotubes</jats:title> <jats:p> The nature of phase transitions changes with system dimensionality. Many aspects of two-dimensional systems have been explored by adsorbing rare gases on graphite surfaces. <jats:bold> Wang <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="552" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1182507">552</jats:related-article> ) reduce the dimensionality further by examining phase transitions of argon and krypton on single-walled carbon nanotubes, following the extent of surface coverage and mapping out phase transitions by using the nanotube as a resonator. Changing the conductance and thus the density of surface electrons also allowed exploration of the effect of adsorbate-surface interactions. </jats:p>

<jats:title>X-rays to Order</jats:title> <jats:p> Self-assembly of molecules is often irreversible. <jats:bold> Cui <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="555" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1182340">555</jats:related-article> , published online 17 December; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5965" page="529" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1185868"> <jats:bold>Safinya and Li</jats:bold> </jats:related-article> </jats:bold> ) examined the ordering of a short peptide sequence (Ala <jats:sub>6</jats:sub> Glu <jats:sub>3</jats:sub> ) terminated with an alkyl chain. Aqueous solutions of this molecule could form hexagonally ordered filaments, but more dilute solutions were disordered. However, prolonged x-ray exposure caused these arrays to become ordered. These arrays were stable for several hours but eventually returned to a disordered state; the addition of salts slowed the ordering processes. It is possible that during the ordering process, X-ray–induced charging affected the repulsive forces that balance tension within the filament. </jats:p>

<jats:title>Packing Puzzle</jats:title> <jats:p> The packing of a large number of spheres is a well-studied problem with maximal packing based on the arrangement of nearest neighbors. With much smaller numbers of particles, it is the free energy that governs which packing arrangements dominate. <jats:bold> Meng <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="560" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1181263">560</jats:related-article> ; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5965" page="535" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1184457">Crocker</jats:related-article> </jats:bold> ) looked at the assembly of colloidal clusters where the number of particles was limited from 2 to 10. For five particles or fewer, only one packing arrangement was found. For six or more particles, while a number of similar energy structures could form, the probability of formation was biased toward those structures with the greater number of nearest-neighbor connections. </jats:p>

<jats:title>Aromatic Silicon</jats:title> <jats:p> Benzene has long intrigued chemists on account of the energy stabilization, termed aromaticity, which arises from π-electron delocalization around its ring framework. A persistent question has been how such stabilization would be impacted were the carbons to be replaced by heavier atoms such as silicon. <jats:bold> Abersfelder <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="564" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1181771">564</jats:related-article> ) have prepared a benzene analog with Si atoms in place of all six-ring carbons, but a slightly altered bonding framework in which substituents outside the ring are no longer evenly distributed. Instead, the substituents pair up at two Si sites, leaving two other ring sites with no external appendages. The resulting compound no longer has a continuous network of π-electrons, but retains a degree of aromatic stabilization involving sigma and nonbonding electrons. </jats:p>

<jats:title>Secondary Selectivity</jats:title> <jats:p> Organic molecules consist principally of rings and chains of methylene (secondary) CH <jats:sub>2</jats:sub> groups, intermittently adorned with oxygen or nitrogen centers and more heavily substituted carbons at the junctions. Synthetic transformations would be most efficient if the C–H bonds in any particular methylene group along the framework could be targeted for selective modification. However, for the most part, these carbon centers prove remarkably hard to differentiate for reactive purposes. <jats:bold>Chen and White</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="566" related-article-type="in-this-issue" vol="327" xlink:href="10.1126/science.1183602">566</jats:related-article> ) now show that an iron catalyst can direct peroxide to oxidize specific secondary C–H bonds preferentially, and with reasonable efficiency, in a range of complex molecules. The observed selectivities follow predictable trends correlated with the electronic and steric environment of the target site. </jats:p>