So-called junk DNA — the 96 per cent of the human genome that seemed to have no useful purpose — may play a more valuable role than its name suggests, U.S. scientists say.

The other four per cent of the genome is made up of genes, which encode for proteins — called "the building blocks of life" by Dr. Michael G. Rosenfeld, of the University of California, San Diego (UCSD) School of Medicine.

Now an international collaboration of scientists led by Rosenfeld has found that some of the junk DNA may serve to create boundaries that help properly organize these building blocks.

"Some of the junk DNA might be considered punctuation marks — commas and periods that help make sense of the coding portion of the genome," said co-author Victoria Lunyak, an assistant research scientist at UCSD.

The researchers, using mice, studied a repeated genomic sequence called SINE B2, which is located on the growth hormone gene locus (the part of a chromosome where a gene is found).

They found that SINE B2 plays a vital role in forming the functional domain boundaries — sections of DNA within the genome that contain all the regulatory signals needed to activate or repress a particular gene.

The researchers say their findings suggest that junk DNA might be the tool mammals use to organize functional domains.

"Without boundary elements, the coding portion of the genome is like a long, run-on sequence of words without punctuation," Rosenfeld said.

The team's work is in the July 13 issue of the journal Science.