Hox proteins reach out round DNA.

well-established techniques for manipulating electron beams in a vacuum. Two decades later the new field of mesoscopic physics arose, in which pieces of metal or semiconductor were made so small and cold that the conduction electrons moved around in them as coherent waves. Fundamental to mesoscopic systems is the interference between a host of different paths available to electrons passing from one point to another. In a magnetic field the Aharonov–Bohm effect should occur between every pair of possible paths, with important but generally rather messy consequences, because the paths are often very complicated and numerous. It was predicted 4 however that, for the particular geometry of a small, thin-walled hollow metallic cylinder in an external magnetic field (Fig. 1b), the Aharonov–Bohm effect should cause the electrical conduc-tance to oscillate with the magnetic flux through the cylinder's bore. When researchers evaporated a normal metal film onto a two-micrometre-thick insulating fibre they found exactly this behaviour 5. Moreover, since then the Aharonov–Bohm effect has become thoroughly established as a basic principle in the physics of mesoscopic systems (many of which, incidentally, are created by drawing patterns with good old electron beams). Nearly two decades further on, meso-scopic devices have now become 'nanostruc-tures' , whose size scales reach down to the molecular level, and which sometimes even employ individual molecules at their heart. Rapidly becoming the archetype of these is a particular class of wire-like molecules with extraordinary electrical properties — the carbon nanotubes 6. The simplest nanotubes are long, flexible, hollow cylinders, each like a drinking straw rolled from a single graphite layer, where the atoms are arranged in a hexagonal lattice. Because these tubes are so narrow (around 1.5 nm in diameter), electrons are quantum mechanically restricted to move only parallel to the tube axis. In the past couple of years this has opened up a real-life laboratory for one-dimensional physics 7,8 , long the preserve of theorists. Another, more common type of nanotube contains many coaxial graphitic cylinders. In these thicker (typically 10-nanometre or more) multiwalled tubes, the electrons can move relatively freely over the outer cylindrical surface — just the thing for observing the Aharonov–Bohm effect. Bachtold et al. 1 attached metal leads to individual multiwalled tubes, and found conductance oscillations consistent with the metallic cylinder theory. Their results dramatically demonstrate the potential for science in such nanostructures, where physics and chemistry merge. The electron-beam paths in the …