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A DNA computer called MAYA has won every game of tic tac toe it has played against human opponents - over 100 in all.

In the classic puzzle, also called noughts and crosses, two players take turns to bag squares of a three-by-three grid, battling to complete the first line. MAYA has a three-by-three array of wells in a plastic laboratory dish, each filled with a unique cocktail of enzymes. Feeding it a particular combination of DNA fragments triggers enzymes in one well to generate a glowing dye.

A game begins when a human player adds magnesium to all of the wells. This fires up the enzymes to play their first move - always the centre square.

Current rules dictate that the human then has to play either the top left corner or left middle, by adding the appropriate DNA fragment to every well - he or she has eight DNA pieces, each corresponding to the eight remaining places on the board. The DNA causes enzymes in another of the squares to glow, and so the game goes on.

MAYA never slips up - it is unbeatable even if its human opponent plays perfectly, in which case there is a draw. It was invented by Milan Stojanovic, of Columbia University in New York, and Darko Stefanovic, of the University of New Mexico in Albuquerque.

A similar approach might be used to create other logic-puzzle computers, says Stefanovic, such as one to solve the 'cannibals and missionaries' conundrum. Here players figure out how to ferry three cannibals and three missionaries across a river, never allowing the hungry to outnumber their prey.

Molecular automatons might one day find medical uses. Injected into the blood, they could recognize molecules on cancerous or healthy cells to deliver a drug accurately. "We don't want to go too deep into games or the [medical] community will not take us seriously," says Stojanovic.