The 46 human chromosomes is where DNA resides. On Wednesday, scientists from around the world reported their findings on a nine-year project to study the 97 percent of the genome that's not, strictly speaking, made up of genes. (National Cancer Institute)

A colossal international effort has yielded the first comprehensive look at how our DNA works, an encyclopedia of information that will rewrite the textbooks and offer new insights into the biology of disease.

For one thing, the effort might help explain why complex diseases such as diabetes, high blood pressure and psychiatric disorders are so difficult to predict and, often, to treat.

The findings, reported Wednesday, reveal that the human genome is packed with at least 4 million on-off switches that tell our genes what to do and when. The switches reside in bits of DNA that once were dismissed as "junk" but turn out to play critical roles in controlling how cells, organs and other tissues behave.

The discovery, considered a major medical and scientific breakthrough, has enormous implications for human health because many complex diseases appear to be caused by tiny changes in hundreds of gene switches.

The findings are the fruit of an immense federal project, involving 440 scientists from 32 labs around the world. As they delved into the "junk" parts of the DNA that are not actual genes containing instructions for proteins they discovered it is not junk. At least 80 percent of it is active and needed.

The result is an annotated road map of much of this DNA, noting what it is doing and how. It includes the system of switches that, acting like dimmer switches for lights, control which genes are used in a cell and when they are used, and determine, for instance, whether a cell becomes a liver cell or a neuron.

The findings have applications for understanding how alterations in the non-gene parts of DNA contribute to disease, which might lead to new drugs.

They can also help explain how the environment can affect disease risk. In the case of identical twins, small changes in environmental exposure can slightly alter gene switches, with the result that one twin gets a disease and the other does not.

"It's Google maps," said Eric Lander, president and founding director of the Broad Institute of Harvard and the Massachusetts Institute of Technology.

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Scientists decode "junk" DNA to show complex inner workings of genes