No spy movie would be complete without at least one roll of microfilm or a few "microdots", miniaturised photographs reduced to the size of a full stop on this page. Microdot technology was used extensively by German spies during the second World War to post innocuous looking letters which carried important but otherwise invisible secrets past the censors.

These elaborate devices are nothing, however, compared to the latest hidden-message technique developed and proven effective by a team of researchers at the Department of Physiology and Biophysics at the Mount Sinai School of Medicine in New York. It used coded strands of synthetic DNA and secreted them amongst the vastness of a human DNA sample, making it impossible to find the message let alone read it.

The DNA in all living things is made up of just four chemicals which bind together in a particular way. It is also blindingly complex given its vast size. Human DNA contains about three billion pairs of these chemicals, strung together in a specific order.

"It occurred to me that this great complexity could be used to hide messages in DNA," stated the team leader at Mount Sinai, Prof Carter Bancroft, professor of physiology and biophysics. "This whole idea is hiding something amongst a large number of similar objects."

He did borrow from the microdot, deciding to develop a secret-message system that used DNA messages, but then reduced them to the size of a full stop that could be hidden on a page and posted through the mail. He described his work in the science journal, Nature. Studying exotic uses for the human genetic blueprint is nothing new for Prof Bancroft.

"I have been active for three years in a new area called DNA-based computation," he explained. This involves trying to use DNA to build a computer. It is based on using DNA as a reactive chemical and not as a blueprint for life.

The DNA microdot technique employs existing and well proven technology but in a wholly novel way. He developed it with the help of Dr Catherine Taylor Clelland and a New York high school student, Ms Viviana Risca. Her involvement is a story in itself.

Prof Bancroft had been judging a science fair in a local school and was very impressed by a project done by Ms Risca.

"I was aware that high school students did sometimes work in science labs for the summer." Her efforts have now made her one of the youngest named authors to be published in peer-reviewed scientific correspondence in Nature. They first established a simple code for the alphabet, using three DNA building blocks for each letter. The word cat for example would be written in DNA code by stringing together nine DNA building blocks in a specific order.

The stringing together of synthetic DNA is a simple matter using a laboratory device known as a synthesiser. The encoded message is also then topped and tailed by special DNA sequences known as primers. These primers enable the message to be recognised and retrieved once hidden.

The great power of the technique only comes into play at this stage. Human DNA is "denatured", broken up into millions of pieces each about 50 to 150 building blocks long. The message is then dropped into this mix where it effectively disappears from view.

All of the mix uses the same four building blocks so the message is indistinguishable from the background material in which it is placed.

To complete the spy thriller aspect of the system, Prof Bancroft then applied tiny amounts of the mix to paper and then cut this up into tiny dots. These were affixed to a letter and posted across the US to confirm that the message could travel through the system and be recovered afterwards.

Bringing back the message was a reverse of the above procedure but with the inclusion of another standard technology made famous by genetic fingerprinting known as polymerase chain reaction (PCR). PCR is what enables forensic scientists to take small samples from a crime scene and then amplify them millions of times, allowing them to match these against samples from a suspect.

The person trying to read the message will know that primers were used to make the message in the first place. PCR is used to produce millions of copies of these primers plus the encoded message between them. These can then be placed in a DNA sequencer and the order of the building blocks read.

A conversion key then allows each three block combination to be translated back into a letter and the message can finally be read.

Prof Bancroft readily admits that the project wasn't all hard work. "First of all it was a lot of fun to do, but I do think it has some potential applications," he said.

There are obvious uses in espionage but also for securing any sensitive information, whether for spies or for companies trying to protect their intellectual assets. Prof Bancroft found that they could reduce the message content to just one part per three million fragments of DNA, making a coding and encryption system virtually impossible to crack. It could also be converted into a powerful way to authenticate things. It could be used to produce passes for workers or tags for clothing that would be impossible to copy or counterfeit. These uses would require the development of devices that could quickly read DNA codes, but the investment could be worthwhile by being able to offer foolproof security.