Dr. Belcher's team uses the virus as a temporary scaffolding on which the crystals grow. The viruses are first altered by the insertion of a few amino acid chains, called peptides, so that they attract a particular material like zinc sulfide or cadmium sulfide.

It is all a matter of affinity, molecular recognition and genetic programming, Dr. Belcher said. "We programmed the virus to grow a particular material at a particular length," she said. "Then we burned off the virus and were left with single-crystal semiconductor wires."