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Reporting this week in Nature1, the researchers, led by Sergio Baranzini at the University of California, San Francisco, and Stephen Kingsmore of the National Center for Genome Resources in Santa Fe, New Mexico, next looked for a difference in epigenetics — chemical modifications to DNA that affect gene expression but not genetic sequence — in the twins' immune cells and in cells of two other sets of similarly affected twins. But no differences were found in the expression levels of key genes, either.

Although they did not sequence the genomes of the other two sets of twins, they did compare 1 million specific 'spelling variations' (known as single nucleotide polymorphisms, or SNPs) in the sequences of twins with and without MS, confirming that their genomes were the same.

Because the study examined the genome so comprehensively, "it is an incredibly important negative", says David Hafler, a neurologist at Yale University in New Haven, Connecticut. The results indicate that there is no clear genetic reason to explain why one twin developed MS while the other did not.