Gazing at an electrical meter, Yi Cui, a graduate student in the Harvard University lab of chemist Charles Lieber, waits for evidence of a remarkable feat in simple, ultrasensitive diagnostics. His target is prostate cancer. His new tool is a microchip bearing 10 silicon wires, each just 10 nanometers (billionths of a meter) wide. These nanowires have been slathered with biological molecules with an affinity for PSA, a protein all too familiar to men of a certain age as the telltale sign of prostate cancer. If the experiment works according to plan, when the PSA molecules bind to the nanowires, there will be a detectable electrical signal.

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