Under three layers of junk, the secret to a fatal brain disease

Writers often compare the human genome to a collection of recipes for making a person. Each gene contains the instructions for building a protein, and our thousands of proteins work together to build and maintain our bodies.

But if the genome is a recipe book, it’s one that was written without a good editor. It is riddled with typos, unnecessary repetitions and meaningless drivel. A miniscule proportion actually codes for proteins. The rest looks like a scrapyard. It contains the remnants of dead genes that are no longer used and have degenerated into nonsense. It contains jumping genes that hop around the genome under their own power, sometimes leaving copies of themselves behind. And it contains the remains of these jumping genes, which have lost their hopping ability and stayed in place.

These “non-coding sequences” are often called junk DNA, and for good reason. It seems that they’re largely useless… but not entirely so. Ever since these non-coding sequences were first discussed, scientists have suspected that some of them play fruitful roles in the body. Many examples have since come to light, and Francois Cartault and his colleagues have found the latest one. He has shown that one piece of supposed “junk” might explain why some people from a tiny French island die from a bizarre brain disease.