Working independently, these three men provided the parts that, when put together, enable the elimination of one gene at a time from the genetic make-up of a mouse.

ECONOMIST: The 2007 Nobel science prizes

But he said that the reasons for humans becoming addicted to any drug were more complex than simply their genetic make-up, and that a test for any gene uncovered by further research would not necessarily work.

BBC: NEWS | Health | Key differences in addicts brains

The genome project, which the government began in 1987, aimed to speed the gene-hunting effort by providing the first full map of all 3 billion base pairs that make up human DNA. (It was not necessary, in this effort, to run through all the pairs on each set of chromosomes, or 6 billion pairs.) And because the base pairs come in only four varieties, they are digital and ideal for computer decoding.

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