U.S. geneticists say a second code hiding within DNA changes how scientists read its instructions and interpret mutations to make sense of health and disease.

Since the genetic code was deciphered in the 1960s, scientists have assumed it was used exclusively to write information about proteins, but University of Washington scientists say they've discovered genomes use the genetic code to write two separate "languages."

One, long understood, describes how proteins are made, while the other instructs the cell on how genes are controlled. One language is written on top of the other, which is why the second language remained hidden for so long, a university release said Thursday.

"For over 40 years we have assumed that DNA changes affecting the genetic code solely impact how proteins are made," UW genome sciences Professor John Stamatoyannopoulos said. "Now we know that this basic assumption about reading the human genome missed half of the picture. These new findings highlight that DNA is an incredibly powerful information storage device, which nature has fully exploited in unexpected ways."

Parts of the genetic code have two meanings, one related to protein sequence, and one related to gene control, the researchers said, and both apparently evolved in concert with each other.

The gene control instructions appear to help stabilize certain beneficial features of proteins and how they are made, they said.

The discovery has major implications for how scientists and physicians interpret a patient's genome and could open new doors to the diagnosis and treatment of disease, Stamatoyannopoulos said.

"The fact that the genetic code can simultaneously write two kinds of information means that many DNA changes that appear to alter protein sequences may actually cause disease by disrupting gene control programs or even both mechanisms simultaneously," he said.