The first maps of the human genome, published this week, show that almost 99 percent is made up of "junk" DNA sequences that do not code for proteins. But one type of junk DNA, called Alu repeats, could help control how other genes are turned on and off, according to molecular biologist Carl Schmid of the University of California, Davis. Schmid's laboratory at UC Davis discovered the Alu repeat in 1979. The new maps show that Alu repeats make up 10 percent of the human genome, and that they are often found near genes that code for proteins. Similar repeats are found in animals from rabbits to silkworms. In 1995, Schmid proposed that Alu repeats are activated when cells are damaged in some way, for example by heat, toxins or lack of essential nutrients. The messages produced by the Alu repeats would help repair cellular damage, by controlling the genes that make proteins. Most scientists have thought that these repeats are parasitic, with no function except to reproduce themselves. The new genome maps lead to more studies of how Alu repeats influence genes, according to Schmid. "Until we resolve this issue, it's impossible to understand genome structure," he said.